FEATURES
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LTC1735
High Efficiency
Synchronous Step-Down
Switching Regulator
DESCRIPTIO
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 generation microprocessors. 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 4V to 30V (36V maximum).
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode and OPTI-LOOP are trademarks of Linear Technology Corporation.
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Synchronizable/Programmable Fixed Frequency
OPTI-LOOP
TM
Compensation Minimizes C
OUT
±
1% Output Voltage Accuracy
Dual N-Channel MOSFET Synchronous Drive
Wide V
IN
Range: 4V to 36V Operation
V
OUT
Range: 0.8V to 6V
Internal Current Foldback
Output Overvoltage Crowbar Protection
Latched Short-Circuit Shutdown Timer
with Defeat Option
Very Low Dropout Operation: 99% Duty Cycle
Forced Continuous Control Pin
Optional Programmable Soft-Start
Remote Output Voltage Sense
Power Good Output (LTC1735F Only)
Logic Controlled Micropower Shutdown: I
Q
< 25µA
LTC1435 Pin Compatible with
Minor Component Changes
Available in 16-Lead Narrow SSOP, SO Packages and
20-Lead TSSOP Package (LTC1735F Only)
APPLICATIO S
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Notebook and Palmtop Computers, PDAs
Cellular Telephones and Wireless Modems
DC Power Distribution Systems
TYPICAL APPLICATIO
C
SS
0.1µF
C
OSC
C
OSC
47pF
TG
RUN/SS
I
TH
C
C2
100pF
BOOST
SW
LTC1735
SGND
V
IN
INTV
CC
BG
PGND
D
B
CMDSH-3
L1
2µH
C
B
0.22µF
M1
FDS6680A
C
IN
22µF
50V
C
C
330pF
R
C
33k
R2
20k
1%
R1
20k
1%
100pF
C
OUT
: PANASONIC EEFUEOG181R
C
IN
: MARCON THCR70E1H226ZT
L1: PANASONIC ETQP6F2R0HFA
R
SENSE
: IRC LRF2010-01-R005J
R
SENSE
0.005Ω
V
OSENSE
SENSE
–
1000pF
SENSE
+
+
4.7µF
M2
FDS6680A
D1
MBRS340T3
Figure 1. High Efficiency Step-Down Converter
1735fc
U
U
U
V
IN
5V TO 24V
V
OUT
1.6V
9A
+
C
OUT
180µF
4V
×
4
SP
1735 F01
1
LTC1735
ABSOLUTE
AXI U RATI GS
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, PGOOD (LTC1735F Only)
Voltages .......................................................7V to – 0.3V
PACKAGE/ORDER I FOR ATIO
TOP VIEW
C
OSC
1
RUN/SS 2
I
TH
3
FCB 4
SGND 5
V
OSENSE
6
SENSE
–
7
SENSE
+
8
GN PACKAGE
16-LEAD NARROW
PLASTIC SSOP
16 TG
15 BOOST
14 SW
13 V
IN
12 INTV
CC
11 BG
10 PGND
9
EXTV
CC
ORDER PART
NUMBER
LTC1735CGN
LTC1735CS
LTC1735IGN
LTC1735IS
LTC1735EGN
GN PART MARKING
1735
1735I
1735E
S PACKAGE
16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/W (GN)
T
JMAX
= 125°C,
θ
JA
= 110°C/W (S)
Consult LTC marketing for parts specified with wider operating temperature ranges.
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
I
VOSENSE
V
OSENSE
∆V
LINEREG
∆V
LOADREG
PARAMETER
Feedback Current
Feedback Voltage
Reference Voltage Line Regulation
Output Voltage Load Regulation
CONDITIONS
(Note 3)
(Note 3)
V
IN
= 3.6V to 30V (Note 3)
(Note 3)
Measured in Servo Loop; V
ITH
= 0.7V
Measured in Servo Loop; V
ITH
= 2V
In Dropout
q
q
q
q
ELECTRICAL CHARACTERISTICS
Main Control Loop
DF Max
g
m
V
FCB
I
FCB
Maximum Duty Factor
Transconductance Amplifier g
m
Forced Continuous Threshold
Forced Continuous Current
V
FCB
= 0.85V
2
U
U
W
W W
U
W
(Note 1)
Peak Driver Output Current <10µs (TG, BG) .............. 3A
INTV
CC
Output Current ......................................... 50mA
Operating Ambient Temperature Range
LTC1735C ............................................... 0°C to 85°C
LTC1735I/LTC1735E (Note 8) ............ – 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
TOP VIEW
NC
C
OSC
RUN/SS
I
TH
FCB
SGND
V
OSENSE
PGOOD
SENSE
–
1
2
3
4
5
6
7
8
9
20 NC
19 TG
18 BOOST
17 SW
16 V
IN
15 INTV
CC
14 BG
13 PGND
12 EXTV
CC
11 NC
ORDER PART
NUMBER
LTC1735CF
LTC1735IF
SENSE
+
10
F PACKAGE
20-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 110°C/W
MIN
TYP
–4
MAX
– 25
0.808
0.02
0.3
– 0.3
UNITS
nA
V
%/V
%
%
%
mmho
0.792
0.8
0.001
0.1
– 0.1
98
0.76
99.4
1.3
0.8
– 0.17
0.84
– 0.3
V
µA
1735fc
LTC1735
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
V
OVL
I
Q
PARAMETER
Feedback Overvoltage Lockout
Input DC Supply Current
Normal Mode
Shutdown
Run Pin Start Threshold
Run Pin Begin Latchoff Threshold
Soft-Start Charge Current
RUN/SS Discharge Current
Undervoltage Lockout
Maximum Current Sense Threshold
Sense Pins Total Source Current
Minimum On-Time
TG Transition Time:
Rise Time
Fall Time
BG Transition Time:
Rise Time
Fall Time
Top Gate Off to Synchronous
Gate On Delay Time
Synchronous Gate Off to Top
Gate On Delay Time
Internal V
CC
Voltage
Internal V
CC
Load Regulation
EXTV
CC
Drop Voltage
EXTV
CC
Switchover Voltage
EXTV
CC
Hysteresis
Oscillator Frequency
Maximum Sync Frequency Ratio
FCB Pin Threshold For Sync
PGOOD Voltage Low
PGOOD Leakage Current
PGOOD Trip Level
Ramping Negative
I
PGOOD
= 2mA
V
PGOOD
= 5V
V
OSENSE
with Respect to Set Output Voltage
V
OSENSE
Ramping Negative
V
OSENSE
Ramping Positive
– 6.0
6.0
– 7.5
7.5
0.9
C
OSC
= 43pF (Note 5)
265
(Note 4)
V
RUN/SS
= 0V
V
RUN/SS
, Ramping Positive
V
RUN/SS
, Ramping Positive
V
RUN/SS
= 0V
Soft Short Condition, V
OSENSE
= 0.5V,
V
RUN/SS
= 4.5V
Measured at V
IN
Pin (V
IN
Ramping Down)
V
OSENSE
= 0.7V
V
SENSE–
= V
SENSE+
= 0V
Tested with a Square Wave (Note 6)
(Note 7)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
(Note 7)
C
LOAD
= 3300pF
C
LOAD
= 3300pF
C
LOAD
= 3300pF Each Driver
C
LOAD
= 3300pF Each Driver
q
q
ELECTRICAL CHARACTERISTICS
CONDITIONS
q
MIN
0.84
TYP
0.86
450
15
MAX
0.88
UNITS
V
µA
µA
V
V
µA
µA
V
mV
µA
ns
ns
ns
ns
ns
ns
ns
25
1.9
4.5
4
3.9
85
80
200
90
90
90
80
V
RUN/SS
V
RUN/SS
I
RUN/SS
I
SCL
UVLO
∆V
SENSE(MAX)
I
SENSE
t
ON(MIN)
TG t
r
TG t
f
BG t
r
BG t
f
TG/BG t
1D
TG/BG t
2D
1.0
– 0.7
0.5
1.5
4.1
– 1.2
2
3.5
60
75
60
160
50
50
50
40
100
70
Internal V
CC
Regulator
V
INTVCC
V
LDO(INT)
V
LDO(EXT)
V
EXTVCC
V
EXTVCC(HYS)
Oscillator
f
OSC
f
H
/f
OSC
f
FCB(SYNC)
V
PGL
I
PGOOD
V
PG
300
1.3
1.2
110
200
±1
– 9.5
9.5
V
mV
µA
%
%
335
kHz
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
CC
= 0 to 20mA, V
EXTVCC
= 4V
I
CC
= 20mA, V
EXTVCC
= 5V
I
CC
= 20mA, EXTV
CC
Ramping Positive
q
5.0
5.2
0.2
130
5.4
1
200
V
%
mV
V
V
4.5
4.7
0.2
PGOOD Output (LTC1735F Only)
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, LTC1735EGN: T
J
= T
A
+ (P
D
• 130°C/W)
LTC1735CF, LTC1735IF: T
J
= T
A
+ (P
D
• 110°C/W)
1735fc
3
LTC1735
ELECTRICAL CHARACTERISTICS
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:
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.
Note 8:
The LTC1735E is guaranteed to meet performance specifications
from 0°C to 85°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls. The LTC1735I specifications are
guaranteed over the full –40°C to 85°C operating temperature range.
f
OSC
–1
8.477(10
11
)
1
1
=
+
C
OSC
(pF )
+
11
I
CHG
I
DIS
TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs Load Current
(3 Operating Modes)
100
90
80
EFFICIENCY (%)
EXTV
CC
OPEN
BURST
EFFICIENCY (%)
70
CONTINUOUS
60
50
40
30
20
0.001
V
IN
= 10V
V
OUT
= 3.3V
R
S
= 0.01Ω
f
O
= 300kHz
0.1
0.01
1
LOAD CURRENT (A)
10
1735 G01
EFFICIENCY (%)
SYNC
Efficiency vs Input Voltage
100
0
EXTV
CC
OPEN
V
OUT
= 1.6V
95 FIGURE 1
90
I
OUT
= 5A
85
I
OUT
= 0.5A
80
75
NORMALIZED V
OUT
(%)
–0.1
EFFICIENCY (%)
V
IN
– V
OUT
(mV)
70
0
5
10
15
20
INPUT VOLTAGE (V)
4
U W
25
1735 G04
Efficiency vs Load Current
100
90
80
V
IN
= 24V
70
60
50
40
0.01
EXTV
CC
= 5V
FIGURE 1
V
IN
= 5V
100
V
IN
= 15V
95
90
Efficiency vs Input Voltage
EXTV
CC
= 5V
V
OUT
= 1.6V
FIGURE 1
I
OUT
= 5A
85
I
OUT
= 0.5A
80
75
70
0.1
1
LOAD CURRENT (A)
10
1735 G02
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1735 G03
Load Regulation
500
FCB = 0V
V
IN
= 15V
FIGURE 1
V
IN
– V
OUT
Dropout Voltage
vs Load Current
R
SENSE
= 0.005Ω
V
OUT
= 5V – 5% DROP
400
300
–0.2
200
–0.3
100
–0.4
0
0
2
6
4
LOAD CURRENT (A)
8
10
1735 G05
30
0
2
4
6
LOAD CURRENT (A)
8
10
1735 G06
1735fc
LTC1735
TYPICAL PERFOR A CE CHARACTERISTICS
Input and Shutdown Currents
vs Input Voltage
500
EXTV
CC
OPEN
400
INPUT CURRENT (µA)
80
100
4
3
2
1
0
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
300
60
EXTV
CC
– INTV
CC
(mV)
INTV
CC
VOLTAGE (V)
200
SHUTDOWN
100
EXTV
CC
= 5V
0
0
5
20
15
10
25
INPUT VOLTAGE (V)
30
35
Maximum Current Sense Threshold
vs Normalized Output Voltage
(Foldback)
80
70
60
50
40
30
20
10
0
0
50
25
75
NORMALIZED OUTPUT VOLTAGE (%)
100
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
V
SENSE(CM)
= 1.6V
MAXIMUM CURRENT SENSE THRESHOLD (mV)
Maximum Current Sense Threshold
vs I
TH
Voltage
MAXIMUM CURRENT SENSE THRESHOLD (mV)
MAXIMUM CURRENT SENSE THRESHOLD (mV)
90
80
70
60
50
40
30
20
10
0
–10
–20
–30
0
0.5
1
1.5
V
ITH
(V)
2
2.5
1735 G13
70
V
ITH
(V)
U W
1735 G07
1735 G10
INTV
CC
Line Regulation
6
5
1mA LOAD
500
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
400
SHUTDOWN CURRENT (µA)
300
40
200
20
100
0
0
0
10
30
40
20
INTV
CC
LOAD CURRENT (mA)
50
1735 G09
1735 G08
Maximum Current Sense Threshold
vs V
RUN/SS
80
80
Maximum Current Sense Threshold
vs Sense Common Mode Voltage
60
76
72
40
68
20
64
0
0
1
2
3
V
RUN/SS
(V)
1735 G11
4
5
6
60
0
1
3
4
2
COMMON MODE VOLTAGE (V)
5
1735 G12
Maximum Current Sense Threshold
vs Temperature
80
2.5
V
SENSE(CM)
= 1.6V
2.0
V
ITH
vs V
RUN/SS
V
OSENSE
= 0.7V
75
1.5
1.0
65
0.5
60
–40 –15
0
85
10
35
60
TEMPERATURE (°C)
110
135
0
1
2
3
V
RUN/SS
(V)
4
5
6
1735 G15
1735 G18
1735fc
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