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1
LTC1735-1
High Efficiency
Synchronous Step-Down
Switching Regulator
The LTC
1735-1 is a synchronous step-down switching
regulator controller optimized for CPU power. OPTI-LOOP
compensation allows the transient response to be opti-
mized over a wide range of output capacitance and ESR
values.
The operating frequency (synchronizable up to 500kHz) is
set by an external capacitor allowing maximum flexibility
in optimizing efficiency. The output voltage is monitored
by a power good window comparator that indicates when
the output is within 7.5% of its programmed value, con-
forming to Intel Mobile CPU Specifications.
Protection features include internal foldback current lim-
iting, output overvoltage crowbar and optional short-
circuit shutdown. Soft-start is provided by an external
capacitor that 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).
Pin defeatable Burst Mode
TM
operation provides high
efficiency at low load currents while 99% duty cycle
provides low dropout operation.
s
Dual N-Channel MOSFET Synchronous Drive
s
Programmable/Synchronizable Fixed Frequency
s
V
OUT
Range: 0.8V to 7V
s
Wide V
IN
Range: 3.5V to 36V Operation
s
Very Low Dropout Operation: 99% Duty Cycle
s
OPTI-LOOP
TM
Compensation Minimizes C
OUT
s
1% Output Voltage Accuracy
s
Power Good Output Voltage Monitor
s
Internal Current Foldback
s
Output Overvoltage Crowbar Protection
s
Latched Short-Circuit Shutdown Timer
with Defeat Option
s
Optional Programmable Soft-Start
s
Remote Output Voltage Sense
s
Logic Controlled Micropower Shutdown: I
Q
< 25
A
s
Available in 16-Lead Narrow SSOP and SO Packages
Figure 1. CPU Core DC/DC Converter with Dynamic Voltage Selection from SpeedStep Enabled Processors
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
Pentium is a registered trademark of Intel Corporation. SpeedStep is a trademark of Intel
Corporation.
s
Notebook and Palmtop Computers, PDAs
s
Power Supply for Mobile Pentium
III Processor with
SpeedStep
TM
Technology
s
Cellular Telephones and Wireless Modems
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
C
OSC
RUN/SS
I
TH
PGOOD
SENSE
SENSE
+
V
OSENSE
SGND
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
EXTV
CC
LTC1735-1
1000pF
C
C2
330pF
C
OSC
47pF
47pF
C
C1
47pF
C
SS
0.1
F
R
C1
33k
+
4.7
F
5V
(OPTIONAL)
47pF
Q4
2N7002
GND
47pF
R1
10k
0.5%
C
IN
: MARCON THCR70E1H226ZT
C
OUT
: PANASONIC EEFUE06181R
L1: PANASONIC ETQP6RZ1R20HFA
R
SENSE
: IRC CRF2010-01-R004J
V
OUT
1.35V TO 1.60V
12A
V
SEL
= 1: V
OUT
= 1.60V
V
SEL
= 0: V
OUT
= 1.35V
D1
CMDSH-3
D2
MBRS340T3
C
B
0.22
F
Q1
FDS6680A
C
IN
22
F
50V
CERAMIC
2
L1
1.2
H
R
SENSE
0.004
V
IN
4.5V TO 24V
PGOOD
Q2, Q3
FDS6680A
2
10
1735-1 F01
R3
33.2k
1%
R2
14.3k
0.5%
10
+
C
OUT
180
F
4V
PANASONIC SP
4
DESCRIPTIO
U
FEATURES
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC1735-1
ABSOLUTE AXI U
RATI GS
W
W
W
U
PACKAGE/ORDER I FOR ATIO
U
U
W
(Note 1)
Input Supply Voltage (V
IN
)........................ 36V to 0.3V
Topside Driver Supply Voltage (BOOST)... 42V to 0.3V
Switch Voltage (SW) ................................... 36V to 5V
INTV
CC
, EXTV
CC
(BOOST, SW) Voltages ..... 7V to 0.3V
SENSE
+
, SENSE
,
PGOOD Voltages ................ 1.1(INTV
CC
+ 0.3V) to 0.3V
I
TH
, V
OSENSE
, C
OSC
Voltages .....................2.7V to 0.3V
RUN/SS Voltage ....................(INTV
CC
+ 0.3V) to 0.3V
Peak Driver Output Current <10
s (TG, BG) .............. 3A
INTV
CC
Output Current ......................................... 50mA
Operating Ambient Temperature Range
LTC1735C-1 ............................................ 0
C to 85
C
LTC1735I-1 ........................................ 40
C to 85
C
Junction Temperature (Note 2) ............................. 125
C
Storage Temperature Range ................. 65
C to 150
C
Lead Temperature (Soldering, 10 sec).................. 300
C
ELECTRICAL CHARACTERISTICS
ORDER PART
NUMBER
LTC1735CGN-1
LTC1735CS-1
LTC1735IGN-1
LTC1735IS-1
Consult factory for Military grade parts.
T
JMAX
= 125
C,
JA
= 130
C/W (GN)
T
JMAX
= 125
C,
JA
= 110
C/W (S)
TOP VIEW
S PACKAGE
16-LEAD PLASTIC SO
GN 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
PGOOD
SENSE
SENSE
+
V
OSENSE
SGND
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
EXTV
CC
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Main Control Loop
I
VOSENSE
Feedback Current
(Note 3)
4
25
nA
V
OSENSE
Feedback Voltage
(Note 3)
q
0.792
0.8
0.808
V
V
LINEREG
Reference Voltage Line Regulation
V
IN
= 3.6V to 30V (Note 3)
0.001
0.02
%/V
V
LOADREG
Output Voltage Load Regulation
(Note 3)
Measured in Servo Loop; V
ITH
= 0.7V
q
0.1
0.3
%
Measured in Servo Loop; V
ITH
= 2V
q
0.1
0.3
%
DF Max
Maximum Duty Factor
In Dropout
98
99.4
%
g
m
Transconductance Amplifier g
m
1.3
mmho
V
OVL
Feedback Overvoltage Lockout
q
0.84
0.86
0.88
V
I
Q
Input DC Supply Current
(Note 5)
Normal Mode
3.6V < V
IN
< 30V
450
A
Shutdown
V
RUN/SS
= 0V
15
25
A
V
RUN/SS
Run Pin Start Threshold
V
RUN/SS
, Ramping Positive
1.0
1.5
1.9
V
Run Pin Begin Latchoff Threshold
V
RUN/SS
, Ramping Positive
4.1
4.5
V
I
RUN/SS
Soft-Start Charge Current
V
RUN/SS
= 0V
0.7
1.2
A
I
SCL
RUN/SS Discharge Current
Soft Short Condition, V
OSENSE
= 0.5V,
0.5
2
4
A
V
RUN/SS
= 4.5V
UVLO
Undervoltage Lockout
Measured at V
IN
Pin (Ramping Negative)
q
3.5
3.9
V
V
SENSE(MAX)
Maximum Current Sense Threshold
V
OSENSE
= 0.7V
q
60
75
85
mV
I
SENSE
SENSE Pins Total Source Current
V
SENSE
= V
SENSE
+
= 0V
60
80
A
t
ON(MIN)
Minimum On-Time
Tested with a Square Wave (Note 4)
160
200
ns
3
LTC1735-1
Note 1: Absolute Maximum Ratings are those values beyond which the life
of the device may be impaired.
Note 2: T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formulas:
LTC1735CS-1, LTC1735IS-1: T
J
= T
A
+ (P
D
110
C/W)
LTC1735CGN-1, LTC1735IGN-1: T
J
= T
A
+ (P
D
130
C/W)
Note 3: The LTC1735-1 is tested in a feedback loop that servos V
OSENSE
to the balance point for the error amplifier (V
ITH
= 1.2V).
Note 4: The minimum on-time condition corresponds to an inductor
peak-to-peak ripple current > 40% of I
MAX
(see Minimum On-Time
Considerations in the Applications Information section).
Note 5: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 6: Oscillator frequency is tested by measuring the C
OSC
charge
current (I
OSC
) and applying the formula:
f
kHz
C
pF
I
I
OSC
OSC
CHG
DIS
(
)
.
(
)
(
)
=
+
+
8 477 10
11
1
1
8
1
Note 7: Rise and fall times are measured using 10% to 90% levels.
Delay times are measured using 50% levels.
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25
C. V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
TG Transition Time:
(Note 7)
TG t
r
Rise Time
C
LOAD
= 3300pF
50
90
ns
TG t
f
Fall Time
C
LOAD
= 3300pF
50
90
ns
BG Transition Time:
(Note 7)
BG t
r
Rise Time
C
LOAD
= 3300pF
50
90
ns
BG t
f
Fall Time
C
LOAD
= 3300pF
40
80
ns
TG/BG T1D
Top Gate Off to Synchronous
C
LOAD
= 3300pF Each Driver
100
ns
Gate-On Delay Time
TG/BG T2D
Synchronous Gate Off to Top
C
LOAD
= 3300pF Each Driver
70
ns
Gate-On Delay Time
Internal V
CC
Regulator
V
INTVCC
Internal V
CC
Voltage
6V < V
IN
< 30V, V
EXTVCC
= 4V
5.0
5.2
5.4
V
V
LDO(INT)
INTV
CC
Load Regulation
I
CC
= 0mA to 20mA, V
EXTVCC
= 4V
0.2
1
%
V
LDO(EXT)
EXTV
CC
Drop Voltage
I
CC
= 20mA, V
EXTVCC
= 5V
130
200
mV
V
EXTVCC
EXTV
CC
Switchover Voltage
I
CC
= 20mA, EXTV
CC
Ramping Positive
q
4.5
4.7
V
V
EXTVCC(HYS)
EXTV
CC
Hysteresis
0.2
V
Oscillator
f
OSC
Oscillator Frequency
(Note 6), C
OSC
= 43pF
265
300
335
kHz
f
H
/f
OSC
Maximum Sync Frequency Ratio
1.3
PGOOD Pin
V
PG(SYNC)
PGOOD Threshold for Sync
Ramping Negative
0.9
1.2
V
V
PG(FC)
PGOOD Threshold for Force Cont.
0.76
0.8
0.84
V
V
PGL
PGOOD Voltage Low
I
PGOOD
= 2mA
110
200
mV
I
PGOOD
PGOOD Pull-Up Current
V
PGOOD
= 0.85V
0.17
A
V
PG
PGOOD Trip Level
V
OSENSE
With Respect to Set Output Voltage
V
OSENSE
Ramping Negative
6.0
7.5
9.5
%
V
OSENSE
Ramping Positive
6.0
7.5
9.5
%
4
LTC1735-1
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Efficiency vs Load Current
(3 Operating Modes)
LOAD CURRENT (A)
0.001
EFFICIENCY (%)
60
70
80
BURST
SYNC
CONT
10
1735-1 G01
50
40
20
0.01
0.1
1
30
100
90
V
IN
= 10V
V
OUT
= 3.3V
R
S
= 0.01
f
O
= 300kHz
EXTV
CC
OPEN
Efficiency vs Load Current
LOAD CURRENT (A)
10mA
100mA
1A
10A
EFFICIENCY (%)
1735-1 G02
100
90
80
70
60
50
40
V
IN
= 5V
EXTV
CC
= 5V
V
OUT
= 1.6V
V
IN
= 24V
V
IN
= 15V
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1735-1 G03
25
30
95
EXTV
CC
= 5V
V
OUT
= 1.6V
FIGURE 1
I
OUT
= 5A
I
OUT
= 0.5A
Efficiency vs Input Voltage
V
IN
V
OUT
Dropout Voltage
vs Load Current
Load Regulation
Efficiency vs Input Voltage
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1735-1 G04
25
30
95
EXTV
CC
OPEN
V
OUT
= 1.6V
FIGURE 1
I
OUT
= 5A
I
OUT
= 0.5A
LOAD CURRENT (A)
0
NORMALIZED V
OUT
(%)
0.2
0.1
8
1735-1 G05
0.3
0.4
2
4
6
10
0
FCB = 0V
V
IN
= 15V
FIGURE 1
LOAD CURRENT (A)
0
0
V
IN
V
OUT
(mV)
500
400
300
200
100
2
4
6
8
1735-1 G06
10
R
SENSE
= 0.005
V
OUT
= 5V 5% DROP
Input and Shutdown Currents
vs Input Voltage
INPUT VOLTAGE (V)
0
5
0
INPUT CURRENT (
A)
SHUTDOWN CURRENT (
A)
200
500
10
20
25
1735-1 G07
100
400
300
0
40
100
20
80
60
15
30
35
EXTV
CC
OPEN
SHUTDOWN
EXTV
CC
= 5V
INTV
CC
Line Regulation
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
INPUT VOLTAGE (V)
0
INTV
CC
VOLTAGE (V)
4
5
6
15
25
1735-1 G08
3
2
5
10
20
30
35
1
0
1mA LOAD
INTV
CC
LOAD CURRENT (mA)
0
EXTV
CC
INTV
CC
(mV)
300
400
500
40
1735-1 G09
200
100
0
10
20
30
50
5
LTC1735-1
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Maximum Current Sense Threshold
vs Normalized Output Voltage
(Foldback)
Maximum Current Sense Threshold
vs V
RUN/SS
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
NORMALIZED OUTPUT VOLTAGE (%)
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
40
50
60
100
1735-1 G10
30
20
0
25
50
75
10
80
70
V
RUN/SS
(V)
0
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
20
40
60
80
1
2
3
4
1735-1 G11
5
6
V
SENSE(CM)
= 1.6V
COMMON MODE VOLTAGE (V)
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
72
76
80
4
1735-1 G12
68
64
60
1
2
3
5
Maximum Current Sense Voltage
vs I
TH
Voltage
V
ITH
(V)
0
MAXIMUM CURRENT SENSE VOLTAGE (V)
30
50
70
90
2
1735-1 G13
10
10
20
40
60
80
0
20
30
0.5
1
1.5
2.5
Output Current vs Duty Cycle
DUTY CYCLE (%)
0
0
AVERAGE OUTPUT CURRENT I
OUT
/I
MAX
(%)
20
40
60
80
100
20
40
60
80
1735-1 G14
100
f
SYNC
= f
O
I
OUT
/I
MAX
(SYNCHRONIZED)
I
OUT
/I
MAX
(FREE RUN)
V
ITH
vs V
RUN/SS
V
RUN/SS
(V)
0
0
V
ITH
(V)
0.5
1.0
1.5
2.0
2.5
1
2
3
4
1735-1 G15
5
6
V
OSENSE
= 0.7V
SENSE Pins Total Source Current
V
SENSE
COMMON MODE VOLTAGE (V)
0
I
SENSE
(
A)
0
1735-1 G16
50
100
2
4
50
100
6
I
TH
Voltage vs Load Current
LOAD CURRENT (A)
0
0
I
TH
VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
1
2
3
4
1735-1 G17
5
6
V
IN
= 10V
V
OUT
= 3.3V
R
SENSE
= 0.01
f
O
= 300kHz
CONTINUOUS
MODE
Burst Mode
OPERATION
SYNCHRONIZED f = f
O
Maximum Current Sense Threshold
vs Temperature
TEMPERATURE (
C)
40
60
MAXIMUM CURRENT SENSE THRESHOLD (mV)
65
70
75
80
15
10
35
60
1735-1 G18
85
110
135
V
SENSE(CM)
= 1.6V
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