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1
LTC1735
High Efficiency
Synchronous Step-Down
Switching Regulator
The LTC
1735 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. The precision 0.8V reference is compatible with
future microprocessor generations. OPTI-LOOP compen-
sation allows the transient response to be optimized 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. A forced continuous control pin
reduces noise and RF interference and can assist second-
ary winding regulation by disabling Burst Mode operation
when the main output is lightly loaded.
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).
s
Dual N-Channel MOSFET Synchronous Drive
s
Synchronizable/Programmable Fixed Frequency
s
Wide V
IN
Range: 3.5V to 36V Operation
s
V
OUT
Range: 0.8V to 6V
s
OPTI-LOOP
TM
Compensation Minimizes C
OUT
s
1% Output Voltage Accuracy
s
Internal Current Foldback
s
Output Overvoltage Crowbar Protection
s
Latched Short-Circuit Shutdown Timer
with Defeat Option
s
Very Low Dropout Operation: 99% Duty Cycle
s
Forced Continuous Control Pin
s
Optional Programmable Soft-Start
s
Remote Output Voltage Sense
s
Logic Controlled Micropower Shutdown: I
Q
< 25
A
s
LTC1435 Pin Compatible with
Minor Component Changes
s
Available in 16-Lead Narrow SSOP and SO Packages
Figure 1. High Efficiency Step-Down Converter
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
s
Notebook and Palmtop Computers, PDAs
s
Cellular Telephones and Wireless Modems
s
DC Power Distribution Systems
C
OSC
RUN/SS
I
TH
SGND
V
OSENSE
SENSE
SENSE
+
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
LTC1735
C
B
0.22
F
100pF
C
C
330pF
R
C
33k
R
SENSE
0.005
V
OUT
1.6V
9A
C
OUT
: PANASONIC EEFUEOG181R
C
IN
: MARCON THCR70E1H226ZT
L1: PANASONIC ETQP6FZR0HFA
R
SENSE
: IRC LRF2010-01-R005J
1000pF
C
SS
0.1
F
C
OSC
47pF
+
4.7
F
+
C
OUT
180
F
4V
4
SP
C
IN
22
F
50V
M1
FDS6680A
M2
FDS6680A
1735 F01
D
B
CMDSH-3
R1
20k
1%
R2
20k
1%
D1
MBRS340T3
V
IN
5V TO 24V
L1
2
H
C
C2
100pF
FEATURES
DESCRIPTIO
U
APPLICATIO S
U
TYPICAL APPLICATIO
U
2
LTC1735
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
EXTV
CC
Voltage ...........................................7V to 0.3V
Boosted Driver Voltage (BOOST SW) .......7V to 0.3V
SENSE
+
, SENSE
Voltages .......... 1.1 (INTV
CC
) to 0.3V
FCB Voltage ............................(INTV
CC
+ 0.3V) to 0.3V
I
TH
, V
OSENSE
Voltages ............................... 2.7V to 0.3V
RUN/SS Voltages .........................................7V to 0.3V
Peak Driver Output Current <10
s (TG, BG) .............. 3A
INTV
CC
Output Current ......................................... 50mA
Operating Ambient Temperature Range
LTC1735C ............................................... 0
C to 85
C
LTC1735I ............................................ 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
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.
ORDER PART
NUMBER
LTC1735CGN
LTC1735CS
LTC1735IGN
LTC1735IS
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 NARROW
PLASTIC SSOP
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
C
OSC
RUN/SS
I
TH
FCB
SGND
V
OSENSE
SENSE
SENSE
+
TG
BOOST
SW
V
IN
INTV
CC
BG
PGND
EXTV
CC
GN PART MARKING
1735
1735I
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
FCB
Forced Continuous Threshold
q
0.76
0.8
0.84
V
I
FCB
Forced Continuous Current
V
FCB
= 0.85V
0.17
0.3
A
V
OVL
Feedback Overvoltage Lockout
q
0.84
0.86
0.88
V
I
Q
Input DC Supply Current
(Note 4)
Normal Mode
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
V
RUN/SS
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 (V
IN
Ramping Down)
q
3.5
3.9
V
V
SENSE(MAX)
Maximum Current Sense Threshold
V
OSENSE
= 0.7V
q
60
75
85
mV
3
LTC1735
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
Note 1: Absolute Maximum Ratings are those values beyond which the life
of a 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, LTC1735IS: T
J
= T
A
+ (P
D
110
C/W)
LTC1735CGN, LTC1735IGN: T
J
= T
A
+ (P
D
130
C/W)
Note 3: The LTC1735 is tested in a feedback loop that servos V
OSENSE
to
the balance point for the error amplifier (V
ITH
= 1.2V).
Note 4: Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
Note 5: Oscillator frequency is tested by measuring the C
OSC
charge
current (I
OSC
) and applying the formula:
f
C
pF
I
I
OSC
OSC
CHG
DIS
=
+
+
8 477 10
11
1
1
11
1
.
(
)
(
)
Note 6: 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 7: Rise and fall times are measured using 10% and 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.
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 6)
160
200
ns
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 t
1D
Top Gate Off to Synchronous
C
LOAD
= 3300pF Each Driver
100
ns
Gate On Delay Time
TG/BG t
2D
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)
Internal V
CC
Load Regulation
I
CC
= 0 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
C
OSC
= 43pF (Note 5)
265
300
335
kHz
f
H
/f
OSC
Maximum Sync Frequency Ratio
1.3
f
FCB(SYNC)
FCB Pin Threshold For Sync
Ramping Negative
0.9
1.2
V
4
LTC1735
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Efficiency vs Load Current
(3 Operating Modes)
Efficiency vs Load Current
Efficiency vs Input Voltage
LOAD CURRENT (A)
0.001
EFFICIENCY (%)
60
70
80
BURST
SYNC
CONT
10
1735 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
LOAD CURRENT (A)
0.01
0.1
1
10
EFFICIENCY (%)
1735 G02
100
90
80
70
60
50
40
V
IN
= 5V
EXTV
CC
= 5V
FIGURE 1
V
IN
= 24V
V
IN
= 15V
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1735 G03
25
30
95
EXTV
CC
= 5V
V
OUT
= 1.6V
FIGURE 1
I
OUT
= 5A
I
OUT
= 0.5A
Efficiency vs Input Voltage
Load Regulation
V
IN
V
OUT
Dropout Voltage
vs Load Current
INPUT VOLTAGE (V)
0
70
EFFICIENCY (%)
75
80
85
90
100
5
10
15
20
1735 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 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 G06
10
R
SENSE
= 0.005
V
OUT
= 5V 5% DROP
Input and Shutdown Currents
vs Input Voltage
INTV
CC
Line Regulation
INPUT VOLTAGE (V)
0
5
0
INPUT CURRENT (
A)
SHUTDOWN CURRENT (
A)
200
500
10
20
25
1735 G07
100
400
300
0
40
100
20
80
60
15
30
35
EXTV
CC
OPEN
SHUTDOWN
EXTV
CC
= 5V
INPUT VOLTAGE (V)
0
INTV
CC
VOLTAGE (V)
4
5
6
15
25
1735 G08
3
2
5
10
20
30
35
1
0
1mA LOAD
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
INTV
CC
LOAD CURRENT (mA)
0
EXTV
CC
INTV
CC
(mV)
300
400
500
40
1735 G09
200
100
0
10
20
30
50
5
LTC1735
Maximum Current Sense Threshold
vs Normalized Output Voltage
(Foldback)
TYPICAL PERFOR A CE CHARACTERISTICS
U
W
Maximum Current Sense Threshold
vs V
RUN/SS
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
Maximum Current Sense Threshold
vs I
TH
Voltage
Output Current vs Duty Cycle
V
ITH
vs V
RUN/SS
SENSE Pins Total Source Current
I
TH
Voltage vs Load Current
Maximum Current Sense Threshold
vs Temperature
NORMALIZED OUTPUT VOLTAGE (%)
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
40
50
60
100
1735 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 G11
5
6
V
SENSE(CM)
= 1.6V
COMMON MODE VOLTAGE (V)
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
72
76
80
4
1735 G12
68
64
60
1
2
3
5
V
ITH
(V)
0
MAXIMUM CURRENT SENSE THRESHOLD (mV)
30
50
70
90
2
1735 G13
10
10
20
40
60
80
0
20
30
0.5
1
1.5
2.5
DUTY CYCLE (%)
0
0
AVERAGE OUTPUT CURRENT I
OUT
/I
MAX
(%)
20
40
60
80
100
20
40
60
80
1735 G14
100
f
SYNC
= f
O
I
OUT
/I
MAX
(SYNC)
I
OUT
/I
MAX
(FREE RUN)
V
RUN/SS
(V)
0
0
V
ITH
(V)
0.5
1.0
1.5
2.0
2.5
1
2
3
4
1735 G15
5
6
V
OSENSE
= 0.7V
V
SENSE
COMMON MODE VOLTAGE (V)
0
I
SENSE
(
A)
0
1735 G16
50
100
2
4
50
100
6
LOAD CURRENT (A)
0
0
I
TH
VOLTAGE (V)
0.5
1.0
1.5
2.0
2.5
1
2
3
4
1735 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
TEMPERATURE (
C)
40
60
MAXIMUM CURRENT SENSE THRESHOLD (mV)
65
70
75
80
15
10
35
60
1735 G18
85
110
135
V
SENSE(CM)
= 1.6V
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