LTC3458
1
3458f
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
High Efficiency: Up to 93%
Inrush Current Limiting and Output Disconnect
Programmable Output Voltages up to 7.5V
1.5V to 6V Input Range
Programmable/Synchronizable Fixed Frequency
Operation up to 1.5MHz
Programmable Automatic Burst Mode
Operation
Current Mode Control with Programmable Soft-Start
Period and Peak Current Limit
700mA at 7V from 5V Input
0.3 N-Channel and 0.4 P-Channel 1.4A Switches
at 5V
OUT
Ultralow Quiescent Currents: 15A Sleep, <1A in
Shutdown
3mm 4mm Thermally Enhanced DFN Package
Point-of-Load Regulators
USB V
BUS
Power
LCD Bias
OLED Displays
1.4A, 1.5MHz Synchronous
Step-Up DC/DC Converter
with Output Disconnect
The LTC
3458 is a high efficiency, current mode, fixed
frequency, step up DC/DC converter with true output
disconnect and inrush current limiting. The LTC3458 is
rated for a 7.5V output and includes a 0.3 N-channel
MOSFET switch and a 0.4 P-channel MOSFET synchro-
nous rectifier. The LTC3458 is well suited for battery
powered applications and includes programmable output
voltage, switching frequency and loop compensation. The
oscillator frequency can be set up to 1.5MHz or synchro-
nized to an external clock.
Quiescent current is only 15A during Burst Mode opera-
tion maximizing battery life in portable applications. The
Burst Mode current threshold, peak current limit, and soft-
start are externally programmable. Other features include
<1A shutdown current, antiringing control, and thermal
limit. The LTC3458 is available in a low profile (0.75mm),
3mm 4mm 12-pin DFN package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
USB to 7V at 1MHz
Burst Mode is a registered trademark of Linear Technology Corporation.
USB to 7V
OUT
V
IN
GND/PGND
SHDN
SYNC
R
T
I
LIM
SW
V
OUT
FB
COMP
SS
BURST
LTC3458
USB
4.35V to 5.25V
2.2F
COEV
10H
DQ7545
ON OFF
200k
124k
133k
316k
33k
0.01F
0.01F
560pF
10pF
10pF
1.5M
22F
X5R
V
OUT
7V
500mA
3458 TA01a
LOAD CURRENT (mA)
EFFICIENCY
POWER LOSS (mW)
100
95
90
85
80
75
70
1000
10
0.1
0.1
10
100
1000
3458 TA01b
1
5.25V
IN
4.35V
IN
POWER LOSS
LTC3458
2
3458f
V
IN
, SS, SYNC Voltages ................................. 0.3 to 7V
BURST, SHDN, V
OUT
Voltages ....................... 0.3 to 8V
Operating Temperature Range
(Notes 2, 3) .........................................40C to 85C
Storage Temperature Range ..................65C to 125C
SW Voltage
DC ........................................................... 0.3V to 8V
Pulsed <100ns ...................................... 0.3V to 10V
ORDER PART
NUMBER
T
JMAX
= 125C,
JA
= 45C/W
LTC3458EDE
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
ELECTRICAL CHARACTERISTICS
The
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25C. V
IN
= 3.3V, V
OUT
= 5V, R
T
= 200k, unless otherwise noted.
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Minimum V
IN
Operating Voltage
T
A
= 0C to 85C
1.4
1.5
V
T
A
= 40C to 0C
1.4
1.7
V
Output Voltage Adjust Range
2.0
7.5
V
Feedback Voltage
0C to 85C, V
OUT
= 3.3V
1.21
1.23
1.25
V
40C to 0C
1.20
1.25
V
Undervoltage (Exit Burst Mode Operation)
Below Feedback Voltage
4%
V
Feedback Input Current
V
FB
= 1.23V
1
50
nA
Quiescent Current - Burst Mode Operation
V
IN
Current at 3.3V
15
30
A
V
OUT
Current at 5V
5
10
A
Quiescent Current - Shutdown
V
IN
Current at 3.3V
0.5
1
A
V
OUT
Current at 0V
1
3
A
Quiescent Current - Active
V
IN
Current Switching
1
3
mA
NMOS Switch Leakage
0.05
5
A
PMOS Switch Leakage
0.05
5
A
NMOS Switch On Resistance
V
OUT
= 5V
0.3
PMOS Switch On Resistance
V
OUT
= 5V
0.4
Fixed NMOS Current Limit
R
ILIM
= 124k
1.4
1.6
A
Maximum Duty Cycle
V
IN
= 3.3V, f
OSC
= 1MHz
80
90
%
Minimum Duty Cycle
0
%
Frequency Accuracy
R
T
= 200k
0.85
1
1.15
MHz
Error Amplifier Transconductance
100
A/V
Error Amplifier Source Current
7
A
Error Amplifier Sink Current
7
A
SYNC Input High
1.5
V
SYNC Input Low
0.35
V
Consult LTC Marketing for parts specified with wider operating temperature ranges.
12
11
10
9
8
7
1
2
3
4
5
6
V
OUT
BURST
SS
GND
COMP
FB
SW
V
IN
SYNC
SHDN
I
LIM
R
T
TOP VIEW
DE12 PACKAGE
12-LEAD (4mm 3mm) PLASTIC DFN
EXPOSED PAD IS PGND (PIN 13),
MUST BE SOLDERED TO PCB
13
DE PART MARKING
3458
LTC3458
3
3458f
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
SHDN Input High
1.25
V
SHDN Input Low
0.3
V
BURST Mode Peak Current
R
ILIM
= 124k
0.4
A
BURST Threshold Voltage
1.10
V
The
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25C. V
IN
= 3.3V, V
OUT
= 5V, R
T
= 200k, unless otherwise noted.
ELECTRICAL CHARACTERISTICS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2: This IC includes overtemperature protection that is intended to
protect the device during momentary overload conditions. Junction
temperature will exceed 125C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3: The LTC3458 is guaranteed to meet performance specifications
from 0C to 70C. Specifications over the 40C to 85C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
TEMPERATURE (C)
TEMPERATURE (C)
V
IN
(V)
40
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
CURRENT (
A)
10
15
60
3458 G03
3458 G01
3458 G02
5
0
15
10
35
85
20
CURRENT (mA)
2000
1800
1600
1400
1200
1000
800
600
400
200
0
200
I
LIMIT
I
BURST PEAK
I
ZERO
I
VIN
I
VOUT
V
OUT
= 7V
L = 10H
R
ILIM
= 124k
V
IN
= 3.3V
V
OUT
= 5V
3458 G04
3458 G05
3458 G06
R
BURST
(k)
50
CURRENT (mA)
200
300
100
150
250
160
140
120
100
80
60
40
20
0
V
IN
(V)
1.5
CURRENT (mA)
250
200
150
100
50
0
3.0
4.0
5.5
2.0
2.5
3.5
4.5
5.0
OSCILLATOR FREQUENCY (kHz)
400
RT (k
)
600
550
500
450
400
350
300
250
200
150
100
1200
600
800
1000
1400
OUT OF
BURST
INTO
BURST
3.3V
OUT
5V
OUT
7.5V
OUT
R
ILIM
= 124k
45
1.4
CURRENT (A)
1.5
1,7
1.8
15
15
30
90
1.6
30
0
45
60
75
R
ILIMIT
= 124k
I
LIMIT
, I
BURST
, T
ZERO
Currents
Burst Mode Quiescent Current
Maximum Load Current in Burst
Oscillator Programming Resistor
Current Limit Accuracy
Typical Burst Mode Threshold and
Hysteresis vs R
BURST
(T
A
= 25C unless otherwise specified)
LTC3458
4
3458f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
3458 G12
3458 G11
3458 G13
3458 G07
3458 G08
FREQUENCY (kHz)
500
EFFICIENCY (%)
95
93
91
89
87
85
1300
700
900
1100
1500
TEMPERATURE (C)
40
R
DS(ON)
(
)
0.5
0.4
0.3
0.2
0.1
0
60
3458 G09
15
10
35
85
V
IN
(V)
MAX LOAD CURRENT (mA)
3458 G10
1200
1000
800
600
400
200
0
1.5
2.5
3.5
4.0
2.0
3.0
4.5
5.0
5.5
3.3V
OUT
5V
OUT
7.5V
OUT
P-CHANNEL
N-CHANNEL
1.8 to 5.5V
IN
at 700kHz
R
ILIM
= 124k
V
IN
= 3.3V
V
OUT
= 5V
V
IN
= 3.3V
V
OUT
= 5V at 100mA
TEMPERATURE (C)
45
0.95
FREQUENCY (MHz)
0.97
1.01
1.03
1.05
15
15
30
90
0.99
30
0
45
60
75
TEMPERATURE (C)
45
15
15
30
90
30
0
45
60
75
TEMPERATURE (C)
45
15
15
30
90
30
0
45
60
75
0.5
VOLTAGE (V)
0.6
0.8
0.9
1.0
0.7
1.20
VOLTAGE (V)
1.21
1.23
1.24
1.25
1.22
TEMPERATURE (C)
45
15
15
30
90
30
0
45
60
75
VOLTAGE (V)
1.5
1.4
1.3
1.2
1.1
1.0
0.9
0.8
OPERATING
SHUTDOWN
RT = 200k
Efficiency vs Frequency
Maximum Load Current
N-Channel and P-Channel R
DS(ON)
(T
A
= 25C unless otherwise specified)
SHDN Pin Threshold and
Hysteresis
SYNC Pin Threshold
FB Voltage
Frequency Accuracy
LTC3458
5
3458f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
SW
2V/DIV
I
L
100mA/DIV
SW
2V/DIV
SW
5V/DIV
I
L
200mA/DIV
V
OUT
100mV/DIV
I
L
200mA/DIV
SW
5V/DIV
V
OUT
100mV/DIV
I
L
200mA/DIV
SS
200mV/DIV
V
OUT
2V/DIV
2V/DIV
V
IN
I
L
200mA/DIV
SW
2V/DIV
I
L
0.5A/DIV
0mA
200ns/DIV
1s/DIV
200ns/DIV
50s/DIV
5ms/DIV
2s/DIV
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
R
ILIM
= 133k
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
C
OUT
= 22F
C
FF
= 22pF
0mA
0mA
0mA
0mA
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
C
OUT
= 22F
C
FF
= 22pF
V
IN
= 3.3V
V
OUT
= 7V
L = 10H
Fixed Frequency (FF)
Discontinuous Current
Fixed Frequency (FF)
Continuous Current
Over-Current with 1.5A I
LIMIT
Burst Mode Operation
Burst Mode Operation Close-Up
Soft-Start into 50 Load
LTC3458
6
3458f
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
FF Mode 100-300mA Load Step
Sync Operation at 1.33MHz
Burst Mode Operation 10mA to
50mA Load Step
Auto Mode 10mA to 100mA Load
Step
10mA to 200mA Load Step
Showing UV Trip
Forced BURST to FF Mode
Switch with 50mA Load
BURST
1V/DIV
V
OUT
200mV/DIV
LOAD
I
L
200mA/DIV
500s/DIV
100mA
10mA
V
OUT
200mV/DIV
V
OUT
200mV/DIV
I
L
200mA/DIV
I
L
500mA/DIV
200s/DIV
200s/DIV
FIXED
FREQUENCY
BURST
4%
SW
5V/DIV
SYNC
2V/DIV
I
L
200mA/DIV
500ns/DIV
COMP
500mV/DIV
BURST
500mV/DIV
V
OUT
200mV/DIV
V
OUT
200mV/DIV
LOAD
I
L
200mA/DIV
I
L
0.5A/DIV
200s/DIV
200s/DIV
50mA
10mA
V
IN
= 3.3V
V
OUT
= 7V
R
OSC
= 200k
V
IN
= 3.3V
V
OUT
= 5V
L = 10H
C
OUT
= 22F
V
IN
= 3.3V
V
OUT
= 5V
L = 10H
C
BURST
= 0.015F
R
BURST
= 133k
V
IN
= 3.3V
V
OUT
= 5V
L = 10H
C
BURST
= 0.015F
R
BURST
= 133k
V
IN
= 3.3V
V
OUT
= 5V
R
Z
= 33K
CC1 = 270pF
CC2 = 10pF
C
OUT
= 22F
0mA
0mA
V
IN
= 3.3V
V
OUT
= 5V
L = 10H
R
Z
= 33k
CC1 = 270pF
CC2 = 10pF
C
OUT
= 22F
L = 10H
F = 1MHz
LTC3458
7
3458f
U
U
U
PI FU CTIO S
SW (Pin 1): Switch Pin for Inductor Connection. During
discontinuous conduction mode an antiring resistor con-
nects SW to V
IN
to reduce noise.
V
IN
(Pin 2): Input Supply Pin. Connect this to the input
supply and decouple with 1F minimum.
SYNC (Pin 3): Oscillator Synchronization Pin. A clock
pulse width of 100ns to 2s is required to synchronize the
internal oscillator. This pin is disabled when grounded.
SHDN (Pin 4): Shutdown Pin. Grounding this pin shuts
down the IC. Connect to >1.25V to enable.
I
LIM
(Pin 5): Adjustable Peak Current Limit. Connect a
resistor from I
LIM
to GND to program the peak inductor
current according to the following formula:
I
R
LIMIT
ILIM
=
200
where I
LIMIT
is in amps and R
T
is in k.
R
T
(Pin 6): Connect a resistor to ground to program the
oscillator frequency, according to the formula:
f
R
OSC
T
=
+
1
0 2 0 004
.
.
where f
OSC
is in MHz and R
T
is in k.
FB (Pin 7): Connect Resistor Divider Tap Here. The output
voltage can be adjusted from 2V to 7.5V. Feedback refer-
ence voltage is typically 1.23V.
COMP (Pin 8): g
m
Error Amp Output. A frequency com-
pensation network is connected from this pin to ground to
compensate the loop. See the section "Compensating the
Feedback Loop" for guidelines.
GND (Pin 9): Signal Ground Pin.
SS (Pin 10): Connect a capacitor between this pin and
ground to set soft-start period. 5A of current is sourced
from SS during soft-start.
t(msec) = C
SS
(F ) 200
BURST (Pin 11): Burst Mode Threshold Adjust Pin. A
resistor/capacitor combination from this pin to ground
programs the average load current at which automatic
Burst Mode operation is entered, according to the formula:
R
I
BURST
BURST
=
10
where R
BURST
is in k and I
BURST
is in amps.
C
C
V
BURST
OUT
OUT
=
,
10 000
where C
BURST(MIN)
and C
OUT
are in F.
To force fixed frequency PWM mode, connect BURST to
V
OUT
through a 50k resistor.
V
OUT
(Pin 12): Output of the Synchronous Rectifier and
Internal Gate Drive Source for the Power Switches.
V
R
R
OUT
=
+
1 23 1
2
1
.
Exposed Pad (PGND) (Pin 13): Must be soldered to PCB
ground, for electrical contact and optimum thermal
performance.
LTC3458
8
3458f
Detailed Description
The LTC3458 provides high efficiency, low noise power
for boost applications with output voltages up to 7.5V. The
true output disconnect feature eliminates inrush current,
and allows V
OUT
to go to zero during shutdown. The
current mode architecture with adaptive slope compensa-
tion provides ease of loop compensation with excellent
transient load response. The low R
DS(ON)
, low gate charge
synchronous switches eliminate the need for an external
Schottky rectifier, and provide efficient high frequency
pulse width modulation (PWM) control. High efficiency is
achieved at light loads when Burst Mode operation is
entered, where the IC's quiescent current is a low 15A
typical on V
IN
. The LTC3458 is designed to provide custom
performance in a variety of applications with program-
mable feedback, current limit, oscillator frequency, soft-
start, and Burst Mode threshold.
BLOCK DIAGRA
W
PGND
PGND
V
BEST
V
BEST
V
SELECT
3
6
9
OSC/SYNC
MAX
DUTY
SYNC
GND
I
LIM
PGND
SS
COMP
SHDN
R
T
V
CC
V
CC
V
CC
SW
V
OUT
FB
BURST
PWM
AND
BURST MODE
DRIVE LOGIC
P-DRIVE
P-DRIVE
P-DRIVE
N-DRIVE
N-DRIVE
N-DRIVE
I
ZERO
DETECT
BURST MODE
CONTROL
FIXED
FREQUENCY
BURST MODE
MUX
SOFT-START
THERMAL SD
4%
UNDERVOLTAGE
REFERENCE/
BIAS
PEAK CURRENT
COMPARATOR
I
COMP
, I
LIMIT
,
I
BURST_PEAK
,
SLOPE COMP
SLEEP
SLEEP
CONTROL
MODE
MODE
MODE
I
COMP/LIMIT_PEAK
I
BURST_PEAK
SLOPE
SLOPE
SLEEP TO
ALL BLOCKS
I_SENSE
BURST ACTIVE
(DISABLED IN
BURST MODE)
+
+
TSD
5
13
10
8
4
11
1
2
12
7
ANTIRING
SW1
UNDER
UNDER
SD
TO ALL BLOCKS
SDB
I
PEAK
CLOCK
BIAS
CURRENTS
UVLO
I
ZERO
3458 BD
ERROR AMPLIFIER/
BURST COMPARATOR
APPLICATIO S I FOR ATIO
W
U
U
U
LTC3458 Programmable Functions
Current Limit/Peak Burst Current. The programmable
current limit circuit sets the maximum peak current in the
internal N-channel MOSFET switch. This clamp level is
programmed using a resistor to ground on I
LIM
. In Burst
Mode operation, the current limit is automatically set to
~1/4 of the programmed current limit for optimal effi-
ciency. A 124k R
ILIM
resistor is recommended in most
applications unless a lower limit is needed to prevent the
external inductor from saturating.
I
R
LIM
=
200
I is in amps and R is in k.
I
I
BURSTPEAK
LIM
1
4
LTC3458
9
3458f
APPLICATIO S I FOR ATIO
W
U
U
U
Current Sensing. Lossless current sensing converts the
peak current signal to a voltage to sum in with the internal
slope compensation. This summed signal is compared to
the error amplifier output to provide a peak current control
command for the PWM. The slope compensation in the IC
is adaptive to the input and output voltage, therefore the
converter provides the proper amount of slope compensa-
tion to ensure stability, but not an excess to cause a loss
of phase margin in the converter.
Output Disconnect and Inrush Limiting. The LTC3458 is
designed to allow true output disconnect by eliminating
body diode conduction of the internal P-channel MOSFET
rectifier. This allows V
0UT
to go to zero volts during
shutdown, drawing no current from the input source. It
also allows for inrush current limiting at turn-on, minimiz-
ing surge currents seen by the input supply. Note that to
obtain the advantages of output disconnect, there must be
no external Schottky diodes connected between SW and
V
OUT
.
Shutdown. The part is shut down by pulling SHDN below
0.3V, and made active by pulling the pin above 1.25V. Note
that SHDN can be driven above V
IN
or V
OUT
, as long as it
is limited to less than 8V.
Synchronous Rectifier. To prevent the inductor current
from running away, the P-channel MOSFET synchronous
rectifier is only enabled when V
OUT
> (V
IN
+ 0.25V).
Thermal Shutdown. If the die temperature reaches ap-
proximately 150C, the part will go into thermal shutdown
and all switches will be turned off and the soft-start
capacitor will be reset. The part will be enabled again when
the die temperature has dropped by 10C (nominal).
Zero Current Amplifier. The zero current amplifier moni-
tors the inductor current to the output and shuts off the
synchronous rectifier once the current is below 50mA
typical, preventing negative inductor current.
Burst Mode Operation
Burst Mode operation can be automatic or user controlled.
In automatic operation, the IC will automatically enter
Burst Mode operation at light load and return to fixed
frequency PWM mode for heavier loads. The user can
program the average load current at which the mode
Error Amp. The error amplifier is a transconductance type,
with its positive input internally connected to the 1.23V
reference, and its negative input connected to FB. A simple
compensation network is placed from COMP to ground.
Internal clamps limit the minimum and maximum error
amp output voltage for improved large signal transient
response. During sleep (in Burst Mode), the compensa-
tion pin is high impedance, however clamps limit the
voltage on the external compensation network, preventing
the compensation capacitor from discharging to zero
during the sleep time.
Oscillator. The frequency of operation is set through a
resistor from R
T
to ground. An internally trimmed timing
capacitor resides inside the IC. The oscillator frequency is
calculated using the following formula:
f
R
OSC
T
=
+
1
0 2 0 004
.
.
where f
OSC
is in MHz and R
T
is in k
The oscillator can be synchronized with an external clock
applied to the SYNC pin. When synchronizing the oscilla-
tor, the free running frequency must be set to approxi-
mately 30% lower than the desired synchronized fre-
quency.
Soft-Start. The soft-start time is programmed with an
external capacitor to ground on SS. An internal current
source charges it with a nominal 5A. The voltage on the
SS pin (in conjunction with the external resistor on I
LIM
) is
used to control the peak current limit until the voltage on
the capacitor exceeds ~1V, at which point the external
resistor sets the peak current. In the event of a com-
manded shutdown, severe short-circuit, or a thermal
shutdown, the capacitor is discharged automatically.
t
(msec)
= C
SS
(F) 200
Other LTC3458 Features and Functions
Antiringing Control. The antiringing control places a
resistor across the inductor to damp the ringing on SW pin
discontinuous conduction mode. The LC ringing
(L = inductor, C
SW
= Capacitance on SW pin) is low energy,
but can cause EMI radiation.
LTC3458
10
3458f
transition occurs using a single resistor. During Burst
Mode operation, the oscillator is shut down, since the on
time is determined by the time it takes the inductor current
to reach a fixed peak current, and the off time is deter-
mined by the time it takes for the inductor current to return
to zero.
In Burst Mode operation, the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
15A of quiescent current. In this mode the output ripple
voltage has a variable frequency component with load
current and will be typically 2% peak-to-peak. This maxi-
mizes efficiency at very light loads by minimizing switch-
ing and quiescent losses. Burst Mode ripple can be re-
duced slightly by using more output capacitance (22F or
greater). This capacitor does not need to be a low ESR type
if low ESR ceramics are also used. Another method of
reducing Burst Mode ripple is to place a small feed-
forward capacitor across the upper resistor in the V
OUT
feedback divider network.
During Burst Mode operation, COMP is disconnected
from the error amplifier in an effort to hold the voltage on
the external compensation network where it was before
entering Burst Mode operation. To minimize the effects of
leakage current and stray resistance, voltage clamps limit
the minimum and maximum voltage on COMP during
Burst Mode operation. This minimizes the transient expe-
rienced when a heavy load is suddenly applied to the
converter after being in Burst Mode operation for an
extended period of time.
For automatic operation, an RC network should be con-
nected from BURST to ground. The value of the resistor
will control the average load current (I
BURST
) at which
Burst Mode operation will be entered and exited (there is
hysteresis to prevent oscillation between modes). The
equation given for the capacitor on BURST is for the
minimum value, to prevent ripple on the BURST pin from
causing the part to oscillate in and out of Burst Mode
operation at the current where the mode transition occurs.
R
I
BURST
BURST
=
10
where R
BURST
is in k and I
BURST
is in amps.
C
C
V
BURST
OUT
OUT
=
,
10 000
where C
BURST(MIN)
and C
OUT
are in F.
Note: the BURST pin only sources current based on
current delivered to V
OUT
through the P-channel MOSFET.
If current in the inductor is allowed to go negative (this can
occur at very light loads and high step-up ratios), the burst
threshold may become inaccurate, preventing the IC from
entering Burst Mode operation. For R
BURST
values greater
than 200k, a larger than recommended inductor value may
be needed to ensure positive inductor current and auto-
matic Burst Mode operation.
In the event that a sudden load transient causes the voltage
level on FB to drop by more than 4% from the regulation
value, an internal pull-up is applied to BURST, forcing the
part quickly out of Burst Mode operation. For optimum
transient response when going between Burst Mode op-
eration and PWM mode, Burst can be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
drop. Note that Burst Mode operation is inhibited during
start-up and soft-start.
Manual Control
For applications requiring fixed frequency operation at all
load currents, connect the BURST pin to V
OUT
through a
50k resistor. To force Burst Mode operation, ground the
BURST pin.
For applications where a large load step can be anticipated,
the circuit below can be used to reduce the voltage
transient on V
OUT
. Automatic operation is achieved when
the external PMOS is off and fixed frequency operation is
commanded when the external PMOS is on. In shutdown,
the PMOS should be off.
APPLICATIO S I FOR ATIO
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Figure 1
0.01F
V
IN
BURST
133k
PMOS
HIGH: AUTO MODE
LOW: FIXED FREQUENCY
3458 FO2
LTC3458
11
3458f
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3458 allows for
the use of small surface mount inductors. Since the
internal slope compensation circuit relies on the inductor's
current slope and frequency, Table 1 should be used to
select an inductor value for a given frequency of operation
( 25%). The recommended value will yield optimal tran-
sient performance while maintaining stable operation.
Inductor values larger than listed in Table 1 are permis-
sible to reduce the current ripple.
Table 1. Recommended Inductor Values
Frequency
Inductor Value(H)
1.5MHz
3.3 to 4.7
1.25MHz
4.7 to 6.8
1MHz
6.8 to 10
750Hz
10 to 15
500kHz
15 to 22
For high efficiency, choose an inductor with high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I
2
R losses, and must be
able to handle the peak inductor current without saturat-
ing. Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents in the
1A to 3A region. To minimize radiated noise, use a
toroidal or shielded inductor. (Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily). See Table 2 for a list of inductor
manufacturers.
Table 2. Inductor Vendor Information
Supplier
Phone
Website
Coilcraft
(847) 639-6400
www.coilcraft.com
TDK
(847) 803-6100
www.component.tdk.com
Murata
USA: (814) 237-1431
(800) 831-9172
www.murata.com
Sumida
USA: (847) 956-0666
Japan: 81-3-3607-5111
www.japanlink.com/sumida
COEV
(800) 227-7040
www.coev.net
Toko
(847) 297-0070
www.tokoam.com
Wurth
(202) 785-8800
www.we-online.com
APPLICATIO S I FOR ATIO
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Some example inductor part types are:
Coilcraft: DO1608 and MSS5131 Series
TDK: RLF5018T and SLF7045 Series
Murata: LQH4C and LQN6C Series
Sumida: CDRH4D28 and CDRH6D28 Series
COEV: DQ7545 Series
TOKO: D62CB and D63LCB Series
WURTH: WE-PD2 Series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
V
I
V
C
V
f
RBULK
P
IN
OUT
OUT
=
where I
P
= peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
V
RCESR
= I
P
C
ESR
where C
ESR
= Capacitor Series Resistance.
The ESL (equivalent series inductance) is also an impor-
tant factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the V
OUT
pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
output voltage ripple. For surface mount applications, AVX
TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo
Yuden X5R type ceramic capacitors are recommended.
For through-hole applications, Sanyo OS-CON capacitors
offer low ESR in a small package size.
In all applications, a minimum of 4.7F (generally 22F is
recommended), low ESR ceramic capacitor should be
placed as close to the V
OUT
pin as possible, and grounded
to a local ground plane.
..
..
LTC3458
12
3458f
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
In most applications >1F per amp of peak input current
is recommended. See Table 3 for a list of capacitor
manufacturers for input and output capacitor selection.
Table 3. Capacitor Vendor Information
Supplier
Phone
Website
AVX
(803) 448 - 9411
www.avxcorp.com
Sanyo
(619) 661 - 6322
www.sanyovideo.com
TDK
(847) 803 - 6100
www.component.tdk.com
Murata
USA: (814) 237-1431
(800) 831-9172
www.murata.com
Taiyo Yuden
(408) 573 - 4150
www.t-yuden.com
Operating Frequency Selection
There are several considerations in selecting the operating
frequency of the converter. The first is staying clear of
sensitive frequency bands, which cannot tolerate any
spectral noise. For example in products incorporating RF
communications the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size of
the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade off is in efficiency, since the switching losses due
to gate charge increase proportional with frequency.
Thermal Considerations
For the LTC3458 to deliver its full output power, it is
imperative that a good thermal path be provided to dissi-
pate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to
conduct heat away from the IC and into a copper plane with
as much area as possible. If the junction temperature rises
above ~150C, the part will go into thermal shutdown, and
all switching will stop until the temperature drops.
Compensating the Feedback Loop
The LTC3458 uses current mode control, with internal
adaptive slope compensation. Current mode control elimi-
nates the 2nd order filter due to the inductor and output
capacitor exhibited in voltage mode controllers, and sim-
plifies the power loop to a single pole filter response. The
product of the modulator control to output DC gain, and
the error amp open-loop gain gives the DC gain of the
system:
G
G
G
V
V
G
G
V
I
G
G
R
DC
CONTROL
EA
REF
OUT
CURRENT SENSE
CONTROL
IN
OUT
EA
CURRENT SENSE
DS ON
=
=
=
,
,
_
_
(
)
2
1000
1
The output filter pole is given by:
f
I
V
C
FILTER POLE
OUT
OUT
OUT
_
,
=
where C
OUT
is the output filter capacitor.
The output filter zero is given by:
f
R
C
FILTER ZERO
ESR
OUT
_
,
=
1
2
where R
ESR
is the output capacitor equivalent series
resistance.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
f
V
I
V
L
RHPZ
IN
OUT
OUT
=
2
2
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
APPLICATIO S I FOR ATIO
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LTC3458
13
3458f
rolled off before the RHP zero frequency.
The typical error amp compensation is shown in Figure 2.
The equations for the loop dynamics are as follows:
f
e
CC
which is close to DC
f
R
CC
f
R
CC
POLE
ZERO
Z
POLE
Z
1
6
1
2
1
2
10
1
1
2
1
1
2
2
=
+
7
8
COMP
FB
ERROR
AMP
1.25V
V
OUT
R1
R2
CC1
CC2
RZ
3458 F01
Figure 2
APPLICATIO S I FOR ATIO
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LTC3458
14
3458f
TYPICAL APPLICATIO S
U
V
IN
GND/PGND
SHDN
SYNC
R
T
I
LIM
SW
V
OUT
FB
COMP
SS
BURST
LTC3458
Li-Ion
2.5V to 4.2V
2.2F
WURTH
12H
774775112
ON OFF
243k
124k
133k
324k
33k
0.01F
0.01F
560pF
10pF
10pF
1M
22F
X5R
V
OUT
5V
450mA
3458 TA03a
V
IN
GND/PGND
SHDN
SYNC
R
T
I
LIM
SW
V
OUT
FB
COMP
SS
BURST
LTC3458
2 ALKALINE
1.8V to 3.3V
2.2F
WURTH
12H
774775112
ON OFF
243k
124k
133k
324k
33k
0.01F
0.01F
560pF
10pF
1M
22F
X5R
V
OUT
5V
200mA
3458 TA04a
10pF
V
IN
GND/PGND
SHDN
SYNC
R
T
I
LIM
SW
V
OUT
FB
COMP
SS
BURST
LTC3458
Li-Ion
2.5V to 4.2V
2.2F
COEV
10H
DQ7545
ON OFF
200k
124k
133k
316k
33k
0.01F
0.01F
560pF
10pF
1.5M
22F
X5R
V
OUT
7V
250mA
3458 TA05a
10pF
Lithium-Ion to 5V, 500mA at 850kHz
Two Cell to 5V
OUT
, 200mA at 850kHz
Lithium-Ion Battery to 7V
OUT
, 250mA at 1MHz
LOAD CURRENT (mA)
EFFICIENCY
100
95
90
85
80
75
70
65
0.1
10
100
1000
3458 TA03b
1
4.2V
IN
3.6V
IN
2.5V
IN
LOAD CURRENT (mA)
EFFICIENCY
100
95
90
85
80
75
70
65
0.1
10
100
1000
3458 TA04b
1
3.3V
IN
1.8V
IN
LOAD CURRENT (mA)
EFFICIENCY
100
95
90
85
80
75
70
65
0.1
10
100
1000
3458 TA05b
1
4.2V
IN
3.6V
IN
2.5V
IN
Li-Ion to 5V
OUT
Two Alkaline to 5V
OUT
Li-Ion to 7V
OUT
LTC3458
15
3458f
4.00 0.10
(2 SIDES)
3.00 0.10
(2 SIDES)
NOTE:
1. DRAWING PROPOSED TO BE A VARIATION OF VERSION
(WGED) IN JEDEC PACKAGE OUTLINE M0-229
2. ALL DIMENSIONS ARE IN MILLIMETERS
3. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE
4. EXPOSED PAD SHALL BE SOLDER PLATED
0.38 0.10
BOTTOM VIEW--EXPOSED PAD
1.70 0.10
(2 SIDES)
0.75 0.05
R = 0.115
TYP
R = 0.20
TYP
0.25 0.05
3.30 0.10
(2 SIDES)
1
6
12
7
0.50
BSC
PIN 1
NOTCH
PIN 1
TOP MARK
0.200 REF
0.00 0.05
(UE12/DE12) DFN 0802
0.25 0.05
3.30 0.05
(2 SIDES)
RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS
1.70 0.05
(2 SIDES)
2.24 0.05
0.50
BSC
0.58 0.05
3.40 0.05
PACKAGE OUTLINE
DE/UE Package
12-Lead Plastic DFN (4mm 3mm)
(Reference LTC DWG # 05-08-1695)
PACKAGE DESCRIPTIO
U
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
LTC3458
16
3458f
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
FAX: (408) 434-0507
www.linear.com
LINEAR TECHNOLOGY CORPORATION 2004
LT/TP 0904 1K PRINTED IN USA
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550mA I
SW
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V
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OUT(MAX)
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SD
< 25A, SO-8,
LT1949-1
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MS8
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1.5A I
SW
, 1.25MHz, High Efficiency Step-Up DC/DC Converter
V
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LTC3400/
600mA I
SW
, 1.2MHz, Synchronous Step-Up DC/DC Converter
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LTC3401
1A I
SW
, 3MHz, Synchronous Step-Up DC/DC Converter
V
IN
: 0.5V to 5V, V
OUT(MAX)
= 6V, I
Q
= 38A I
SD
< 1A, MS10
LTC3402
2A I
SW
, 3MHz, Synchronous Step-Up DC/DC Converter
V
IN
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< 1A, MS10
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SW
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LTC3429
600mA, 500kHz, Synchronous Step-Up DC/DC Converter
V
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< 1A,
with Output Disconnect and Soft-Start
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SW
, 10V Micropower Synchronous Boost/Output Disconnect
V
IN
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LT3460
320mA I
SW
, 1.3MHz, High Efficiency Step-Up DC/DC Converter
V
IN
: 2.5V to 16V, V
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= 36V, I
Q
= 2mA, I
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< 1A, SC70,
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85mA I
SW
, Constant Off-Time, High Efficiency Step-Up DC/DC
V
IN
: 2.3V to 10V, V
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Converter with Integrated Schottky/Output Disconnect
ThinSOT
Dual Lumiled Application with BURST Pin Current Regulation
V
IN
GND/PGND
SHDN
SYNC
R
T
I
LIM
SW
V
OUT
FB
COMP
SS
BURST
LTC3458
Li-Ion
2.7V to 4.2V
C
IN
2.2F
L1
Z1
D1
D2
ON OFF
243k
124k
R
BURST
33k
0.01F
0.01F
0.01F
C
OUT
2.2F
V
OUT
6.4V TO 6.8V
3458 TA06a
C
IN
, C
OUT
: TAIYO YUDEN JMK107BJ225MA
D1, D2: LUXEON EMITTER LUMILED WHITE
LXHLMW1D (2.9V AT 350mA)
L1: Wurth 12H 774775112
R
BURST
: 35.7k FOR 350mA,
47.5k FOR 250mA,
82.5k FOR 150mA
Z1: CENTRAL SEMI 6.8V ZENER DIODE SOT-23 CMPZ5235B
f
OSC
= 850kHz
INPUT VOLTAGE (V)
EFFICIENCY (%)
100
90
80
70
60
50
2.5
3.5
4.0
3458 TA06b
2.0
3.0
4.5
5.0
5.5
350mA, 6.8V
150mA, 6.4V
250mA, 6.6V
NOTE: LUMILED CURRENT REGULATION
~10% OVER V
IN
RANGE
2-Lumileds in Series
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