1
LTC1435
High Efficiency Low Noise
Synchronous Step-Down
Switching Regulator
Figure 1. High Efficiency Step-Down Converter
FEATURES
DESCRIPTIO
N
U
s
Dual N-Channel MOSFET Synchronous Drive
s
Programmable Fixed Frequency
s
Wide V
IN
Range: 3.5V to 36V Operation
s
Ultrahigh Efficiency
s
Very Low Dropout Operation: 99% Duty Cycle
s
Low Standby Current
s
Secondary Feedback Control
s
Programmable Soft Start
s
Remote Output Voltage Sense
s
Logic Controlled Micropower Shutdown: I
Q
< 25
A
s
Foldback Current Limiting (Optional)
s
Current Mode Operation for Excellent Line and Load
Transient Response
s
Output Voltages from 1.19V to 9V
s
Available in 16-Lead Narrow SO and SSOP Packages
The LTC
1435 is a synchronous step-down switching
regulator controller that drives external N-channel power
MOSFETs using a fixed frequency architecture. Burst
Mode
TM
operation provides high efficiency at low load
currents. A maximum duty cycle limit of 99% provides low
dropout operation which extends operating time in bat-
tery-operated systems.
The operating frequency is set by an external capacitor
allowing maximum flexibility in optimizing efficiency. A
secondary winding feedback control pin, SFB, guarantees
regulation regardless of load on the main output by
forcing continuous operation. Burst Mode operation is
inhibited when the SFB pin is pulled low which reduces
noise and RF interference.
Soft start is provided by an external capacitor which can
be used to properly sequence supplies. The operating
current level is user-programmable via an external current
sense resistor. Wide input supply range allows operation
from 3.5V to 30V (36V maximum).
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
s
Notebook and Palmtop Computers, PDAs
s
Cellular Telephones and Wireless Modems
s
Portable Instruments
s
Battery-Operated Devices
s
DC Power Distribution Systems
APPLICATIO
N
S
U
TYPICAL APPLICATIO
N
U
I
TH
TG
C
OSC
68pF
C
C
330pF
100pF
R
C
10k
C
SS
0.1
F
SENSE
SENSE
+
LTC1435
1000pF
+
4.7
F
D
B
CMDSH-3
C
B
0.1
F
M2
Si4412DY
D1
MBRS140T3
L1
10
H
M1
Si4412DY
R
SENSE
0.033
+
C
IN
22
F
35V
2
V
IN
4.5V TO 28V
+
C
OUT
100
F
10V
2
R1
32.4k
R2
22.1k
V
OUT
2.9V/3.5A
PGND
BG
C
OSC
V
IN
V
OSENSE
BOOST
1435 F01
SGND
RUN/SS
SW
INTV
CC
2
LTC1435
ABSOLUTE
M
AXI
M
U
M
RATINGS
W
W
W
U
PACKAGE/ORDER I
N
FOR
M
ATIO
N
W
U
U
ORDER PART
NUMBER
Input Supply Voltage (V
IN
)......................... 36V to 0.3V
Topside Driver Supply Voltage (Boost) ...... 42V to 0.3V
Switch Voltage (SW)............................. V
IN
+ 5V to 5V
EXTV
CC
Voltage ........................................ 10V to 0.3V
Sense
+
, Sense
Voltages ......... INTV
CC
+ 0.3V to 0.3V
I
TH
, V
OSENSE
Voltages .............................. 2.7V to 0.3V
SFB, Run/SS Voltages .............................. 10V to 0.3V
Peak Driver Output Current < 10
s (TG, BG) ............. 2A
INTV
CC
Output Current ........................................ 50mA
Operating Ambient Temperature Range
LTC1435C............................................... 0
C to 70
C
LTC1435I ............................................ 40
C to 85
C
Junction Temperature (Note 1)............................. 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
LTC1435CG
LTC1435CS
LTC1435IG
LTC1435IS
T
A
= 25
C, V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
I
IN
V
OSENSE
Feedback Current
(Note 2)
10
50
nA
V
OSENSE
Feedback Voltage
(Note 2)
q
1.178
1.19
1.202
V
V
LINEREG
Reference Voltage Line Regulation
V
IN
= 3.6V to 20V (Note 2)
0.002
0.01
%/V
V
LOADREG
Output Voltage Load Regulation
I
TH
Sinking 5
A (Note 2)
q
0.5
0.8
%
I
TH
Sourcing 5
A
q
0.5
0.8
%
V
SFB
Secondary Feedback Threshold
V
SFB
Ramping Negative
q
1.16
1.19
1.22
V
I
SFB
Secondary Feedback Current
V
SFB
= 1.5V
1
2
A
V
OVL
Output Overvoltage Lockout
1.24
1.28
1.32
V
I
Q
Input DC Supply Current
EXTV
CC
= 5V (Note 3)
Normal Mode
3.6V < V
IN
< 30V
260
A
Shutdown
V
RUN/SS
= 0V, 3.6V < V
IN
< 15V
16
25
A
V
RUN/SS
Run Pin Threshold
q
0.8
1.3
2
V
I
RUN/SS
Soft Start Current Source
V
RUN/SS
= 0V
1.5
3
4.5
A
V
SENSE(MAX)
Maximum Current Sense Threshold
V
OSENSE
= 0V, 5V
130
150
180
mV
TG Transition Time
TG t
r
Rise Time
C
LOAD
= 3000pF
50
150
ns
TG t
f
Fall Time
C
LOAD
= 3000pF
50
150
ns
BG Transition Time
BG t
r
Rise Time
C
LOAD
= 3000pF
50
150
ns
BG t
f
Fall Time
C
LOAD
= 3000pF
40
150
ns
Internal V
CC
Regulator
V
INTVCC
Internal V
CC
Voltage
6V < V
IN
< 30V, V
EXTVCC
= 4V
q
4.8
5.0
5.2
V
V
LDO
INT
INTV
CC
Load Regulation
I
INTVCC
= 15mA, V
EXTVCC
= 4V
0.2
1
%
V
LDO
EXT
EXTV
CC
Voltage Drop
I
INTVCC
= 15mA, V
EXTVCC
= 5V
130
230
mV
V
EXTVCC
EXTV
CC
Switchover Voltage
I
INTVCC
= 15mA, V
EXTVCC
Ramping Positive
q
4.5
4.7
V
Oscillator
f
OSC
Oscillator Frequency
C
OSC
= 100pF (Note 4)
112
125
138
kHz
TOP VIEW
S PACKAGE
16-LEAD PLASTIC SO
G PACKAGE
16-LEAD PLASTIC SSOP
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
C
OSC
RUN/SS
I
TH
SFB
SGND
V
OSENSE
SENSE
SENSE
+
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
EXTV
CC
T
JMAX
= 125
C,
JA
= 130
C/ W (G)
T
JMAX
= 125
C,
JA
= 110
C/ W (S)
Consult factory for Military grade parts.
3
LTC1435
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range.
LTC1435CG/LTC1435CS: 0
C
T
A
70
C
LTC1435IG/LTC1435IS: 40
C
T
A
85
C
Note 1: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
LTC1435CG/LTC1435IG: T
J
= T
A
+ (P
D
)(130
C/W)
LTC1435CS/LTC1435IS: T
J
= T
A
+ (P
D
)(110
C/W)
Note 2: The LTC1435 is tested in a feedback loop which servos V
OSENSE
to the balance point for the error amplifier (V
ITH
= 1.19V).
Note 3: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 4: Oscillator frequency is tested by measuring the C
OSC
charge and
discharge currents and applying the formula:
f
OSC
(kHz) =
+
1
8.4(10
8
)
C
OSC
(pF) + 11
(
)
1
I
CHG
(
)
1
I
DIS
T
A
= 25
C, V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1435 G01
25
30
95
I
LOAD
= 1A
I
LOAD
= 100mA
V
OUT
= 3.3V
Efficiency vs Input Voltage
V
OUT
= 3.3V
V
IN
V
OUT
Dropout Voltage
vs Load Current
LOAD CURRENT (A)
0
0
V
IN
V
OUT
(V)
0.2
0.1
0.3
0.4
0.5
0.5
1.0
1.5
2.0
1435 G04
2.5
3.0
R
SENSE
= 0.033
V
OUT
DROP OF 5%
LOAD CURRENT (A)
0.001
50
EFFICIENCY (%)
55
65
70
75
100
85
0.01
0.1
1
1435 G03
60
90
95
80
10
CONTINUOUS
MODE
V
IN
= 10V
V
OUT
= 5V
R
SENSE
= 0.033
Burst Mode
OPERATION
Efficiency vs Load Current
V
ITH
Pin Voltage vs Output Current
OUTPUT CURRENT (%)
0
V
ITH
(V)
1.0
2.0
3.0
0.5
1.5
2.5
20
40
60
80
1435 G06
100
10
0
30
50
70
90
Burst Mode
OPERATION
CONTINUOUS
MODE
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1435 G02
25
30
95
I
LOAD
= 1A
I
LOAD
= 100mA
V
OUT
= 5V
Efficiency vs Input Voltage
V
OUT
= 5V
Load Regulation
LOAD CURRENT (A)
0
V
OUT
(%)
0
0.5
1.0
1.5
2.0
1435 G05
2.5
3.0
0.25
0.50
0.75
1.00
1.25
1.50
R
SENSE
= 0.033
4
LTC1435
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Input Supply and Shutdown
Current vs Input Voltage
INPUT VOLTAGE (V)
0
0
SUPPLY CURRENT (mA)
SHUTDOWN CURRENT (
A)
0.5
1.0
1.5
2.0
2.5
0
20
40
60
80
100
5
10
15
20
1435 G07
25
30
V
OUT
= 3.3V
EXTV
CC
= OPEN
V
OUT
= 5V
EXTV
CC
= V
OUT
SHUTDOWN
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
INTV
CC
LOAD CURRENT (mA)
0
EXTV
CC
INTV
CC
(mV)
120
160
200
16
1435 G09
80
40
100
140
180
60
20
0
4
8
12
2
18
6
10
14
20
55
C
25
C
70
C
INTV
CC
LOAD CURRENT (mA)
0
INTV
CC
(%)
0
0.3
20
1435 G08
0.3
0.5
5
10
70
C
25
C
15
0.5
V
EXTVCC
= 0V
INTV
CC
Regulation
vs INTV
CC
Load Current
Normalized Oscillator Frequency
vs Temperature
TEMPERATURE (
C)
40
FREQUENCY (%)
5
10
35
85
1435 G10
f
O
15
10
60
110
135
5
10
RUN/SS Pin Current
vs Temperature
TEMPERATURE (
C)
40
0
RUN/SS CURRENT (
A)
1
2
3
4
15
10
35
60
1435 G11
85
110
135
SFB Pin Current vs Temperature
TEMPERATURE (
C)
40
SFB CURRENT (
A) 0.50
0.25
0
35
85
1435 G12
0.75
1.00
15
10
60
110
135
1.25
1.50
TEMPERATURE (
C)
40
146
CURRENT SENSE THRESHOLD (mV)
148
150
152
154
15
10
35
60
1435 G13
85
110
135
Transient Response
Transient Response
Maximum Current Sense
Threshold Voltage vs Temperature
V
OUT
50mV/DIV
I
LOAD
= 50mA to 1A
1435 G14
I
LOAD
= 1A to 3A
1435 G15
V
OUT
50mV/DIV
5
LTC1435
TYPICAL PERFOR
M
A
N
CE CHARACTERISTICS
U
W
Soft Start: Load Current vs Time
Burst Mode Operation
PI
N
FU
N
CTIO
N
S
U
U
U
ever EXTV
CC
is higher than 4.7V. See EXTV
CC
connection
in Applications Information section. Do not exceed 10V on
this pin. Connect to V
OUT
if V
OUT
5V.
PGND (Pin 10): Driver Power Ground. Connects to source
of bottom N-channel MOSFET and the () terminal of C
IN
.
BG (Pin 11): High Current Gate Drive for Bottom
N-Channel MOSFET. Voltage swing at this pin is from
ground to INTV
CC
.
INTV
CC
(Pin 12): Output of the Internal 5V Regulator and
EXTV
CC
Switch. The driver and control circuits are pow-
ered from this voltage. Must be closely decoupled to power
ground with a minimum of 2.2
F tantalum or electrolytic
capacitor.
V
IN
(Pin 13): Main Supply Pin. Must be closely decoupled
to the IC's signal ground pin.
SW (Pin 14): Switch Node Connection to Inductor. Volt-
age swing at this pin is from a Schottky diode (external)
voltage drop below ground to V
IN
.
BOOST (Pin 15): Supply to Topside Floating Driver. The
bootstrap capacitor is returned to this pin. Voltage swing
at this pin is from INTV
CC
to V
IN
+ INTV
CC
.
TG (Pin 16): High Current Gate Drive for Top N-Channel
MOSFET. This is the output of a floating driver with a
voltage swing equal to INTV
CC
superimposed on the
switch node voltage SW.
C
OSC
(Pin 1): External capacitor C
OSC
from this pin to
ground sets the operating frequency.
RUN/SS (Pin 2): Combination of Soft Start and Run
Control Inputs. A capacitor to ground at this pin sets the
ramp time to full current output. The time is approximately
0.5s/
F. Forcing this pin below 1.3V causes the device to
be shut down. In shutdown all functions are disabled.
I
TH
(Pin 3): Error Amplifier Compensation Point. The
current comparator threshold increases with this control
voltage. Nominal voltage range for this pin is 0V to 2.5V.
SFB (Pin 4): Secondary Winding Feedback Input. Nor-
mally connected to a feedback resistive divider from the
secondary winding. This pin should be tied to: ground to
force continuous operation; INTV
CC
in applications that
don't use a secondary winding; and a resistive divider from
the output in applications using a secondary winding.
SGND (Pin 5): Small-Signal Ground. Must be routed
separately from other grounds to the () terminal of C
OUT
.
V
OSENSE
(Pin 6): Receives the feedback voltage from an
external resistive divider across the output.
SENSE
(Pin 7): The () Input to the Current Comparator.
SENSE
+
(Pin 8): The (+) Input to the Current Comparator.
Built-in offsets between SENSE
and SENSE
+
pins in
conjunction with R
SENSE
set the current trip thresholds.
EXTV
CC
(Pin 9): Input to the Internal Switch Connected to
INTV
CC
. This switch closes and supplies V
CC
power when-
I
LOAD
= 50mA
1435 G16
V
OUT
20mV/DIV
V
ITH
200mV/DIV
1435 G17
RUN/SS
5V/DIV
INDUCTOR
CURRENT
1A/DIV
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