LTC1435
High Efficiency Low Noise
Synchronous Step-Down
Switching Regulator
FEATURES
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DESCRIPTION
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.
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Dual N-Channel MOSFET Synchronous Drive
Programmable Fixed Frequency
Wide V
IN
Range: 3.5V to 36V Operation
Ultrahigh Efficiency
Very Low Dropout Operation: 99% Duty Cycle
Low Standby Current
Secondary Feedback Control
Programmable Soft Start
Remote Output Voltage Sense
Logic Controlled Micropower Shutdown: I
Q
< 25µA
Foldback Current Limiting (Optional)
Current Mode Operation for Excellent Line and Load
Transient Response
Output Voltages from 1.19V to 9V
Available in 16-Lead Narrow SO and SSOP Packages
APPLICATIONS
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Notebook and Palmtop Computers, PDAs
Cellular Telephones and Wireless Modems
Portable Instruments
Battery-Operated Devices
DC Power Distribution Systems
TYPICAL APPLICATION
C
OSC
68pF
C
OSC
C
SS
0.1µF
C
C
330pF
R
C
10k
SGND
100pF
V
OSENSE
SENSE
–
BG
PGND
SENSE
+
RUN/SS
I
TH
LTC1435
INTV
CC
BOOST
V
IN
TG
SW
V
IN
4.5V TO 28V
M1
Si4412DY
+
C
IN
22µF
35V
×
2
D
B
CMDSH-3
C
B
0.1µF
L1
10µH
+
4.7µF
M2
Si4412DY
D1
MBRS140T3
1000pF
Figure 1. High Efficiency Step-Down Converter
U
U
U
R
SENSE
0.033Ω
V
OUT
2.9V/3.5A
R1
32.4k
R2
22.1k
C
OUT
+
100µF
10V
×
2
1435 F01
1
LTC1435
ABSOLUTE
MAXIMUM
RATINGS
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 < 10µs (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
PACKAGE/ORDER INFORMATION
TOP VIEW
C
OSC
1
RUN/SS 2
I
TH
3
SFB 4
SGND 5
V
OSENSE
6
SENSE
–
7
SENSE
+
8
16 TG
15 BOOST
14 SW
13 V
IN
12 INTV
CC
11 BG
10 PGND
9
EXTV
CC
ORDER PART
NUMBER
LTC1435CG
LTC1435CS
LTC1435IG
LTC1435IS
G PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 130°C/ W (G)
T
JMAX
= 125°C,
θ
JA
= 110°C/ W (S)
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
Main Control Loop
I
IN
V
OSENSE
Feedback Current
V
OSENSE
Feedback Voltage
∆V
LINEREG
Reference Voltage Line Regulation
∆V
LOADREG
Output Voltage Load Regulation
Secondary Feedback Threshold
Secondary Feedback Current
Output Overvoltage Lockout
Input DC Supply Current
Normal Mode
Shutdown
V
RUN/SS
Run Pin Threshold
I
RUN/SS
Soft Start Current Source
∆V
SENSE(MAX)
Maximum Current Sense Threshold
TG Transition Time
Rise Time
TG t
r
TG t
f
Fall Time
BG Transition Time
Rise Time
BG t
r
BG t
f
Fall Time
Internal V
CC
Regulator
V
INTVCC
Internal V
CC
Voltage
V
LDO
INT
INTV
CC
Load Regulation
V
LDO
EXT
EXTV
CC
Voltage Drop
V
EXTVCC
EXTV
CC
Switchover Voltage
Oscillator
f
OSC
Oscillator Frequency
V
SFB
I
SFB
V
OVL
I
Q
T
A
= 25°C, V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
MIN
TYP
10
1.19
0.002
0.5
– 0.5
1.19
–1
1.28
260
16
1.3
3
150
50
50
50
40
q
CONDITIONS
(Note 2)
(Note 2)
V
IN
= 3.6V to 20V (Note 2)
I
TH
Sinking 5µA (Note 2)
I
TH
Sourcing 5µA
V
SFB
Ramping Negative
V
SFB
= 1.5V
EXTV
CC
= 5V (Note 3)
3.6V < V
IN
< 30V
V
RUN/SS
= 0V, 3.6V < V
IN
< 15V
q
MAX
50
1.202
0.01
0.8
– 0.8
1.22
–2
1.32
UNITS
nA
V
%/V
%
%
V
µA
V
µA
µA
V
µA
mV
ns
ns
ns
ns
V
%
mV
V
kHz
q
q
q
q
1.178
1.16
1.24
V
RUN/SS
= 0V
V
OSENSE
= 0V, 5V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
C
LOAD
= 3000pF
C
LOAD
= 3000pF
6V < V
IN
< 30V, V
EXTVCC
= 4V
I
INTVCC
= 15mA, V
EXTVCC
= 4V
I
INTVCC
= 15mA, V
EXTVCC
= 5V
I
INTVCC
= 15mA, V
EXTVCC
Ramping Positive
C
OSC
= 100pF (Note 4)
0.8
1.5
130
25
2
4.5
180
150
150
150
150
5.2
–1
230
4.8
q
4.5
112
5.0
– 0.2
130
4.7
125
138
2
U
W
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W W
W
LTC1435
ELECTRICAL CHARACTERISTICS
T
A
= 25°C, V
IN
= 15V, V
RUN/SS
= 5V unless otherwise noted.
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:
8.4(10
8
)
1 + 1
–1
f
OSC
(kHz) = C
OSC
(pF) + 11 I
CHG
I
DIS
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)
(
)(
)
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Input Voltage
V
OUT
= 3.3V
100
V
OUT
= 3.3V
95
I
LOAD
= 1A
EFFICIENCY (%)
EFFICIENCY (%)
90
85
I
LOAD
= 100mA
80
75
70
90
I
LOAD
= 100mA
85
80
75
70
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1435 G01
EFFICIENCY (%)
V
IN
– V
OUT
Dropout Voltage
vs Load Current
0.5
0
R
SENSE
= 0.033Ω
V
OUT
DROP OF 5%
0.4
V
IN
– V
OUT
(V)
– 0.75
–1.00
V
ITH
(V)
0.3
∆V
OUT
(%)
0.2
0.1
0
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3.0
1435 G04
U W
Efficiency vs Input Voltage
V
OUT
= 5V
100
V
OUT
= 5V
95
I
LOAD
= 1A
100
95
90
85
80
75
70
65
60
55
Efficiency vs Load Current
V
IN
= 10V
V
OUT
= 5V
R
SENSE
= 0.033Ω
Burst Mode
OPERATION
CONTINUOUS
MODE
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1435 G02
50
0.001
1
0.01
0.1
LOAD CURRENT (A)
10
1435 G03
Load Regulation
3.0
R
SENSE
= 0.033Ω
– 0.25
– 0.50
V
ITH
Pin Voltage vs Output Current
2.5
2.0
1.5
Burst Mode
OPERATION
1.0
0.5
0
CONTINUOUS
MODE
–1.25
–1.50
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3.0
1435 G05
0
10 20 30 40 50 60 70 80 90 100
OUTPUT CURRENT (%)
1435 G06
3
LTC1435
TYPICAL PERFORMANCE CHARACTERISTICS
Input Supply and Shutdown
Current vs Input Voltage
2.5
100
0.5
2.0
SUPPLY CURRENT (mA)
EXTV
CC
– INTV
CC
(mV)
∆INTV
CC
(%)
1.5
V
OUT
= 5V
EXTV
CC
= V
OUT
V
OUT
= 3.3V
EXTV
CC
= OPEN
1.0
0.5
SHUTDOWN
0
0
5
10
15
20
INPUT VOLTAGE (V)
25
30
1435 G07
Normalized Oscillator Frequency
vs Temperature
10
4
RUN/SS CURRENT (µA)
5
FREQUENCY (%)
SFB CURRENT (µA)
f
O
–5
–10
– 40 –15
60
35
85
10
TEMPERATURE (°C)
Maximum Current Sense
Threshold Voltage vs Temperature
154
CURRENT SENSE THRESHOLD (mV)
152
V
OUT
50mV/DIV
150
V
OUT
50mV/DIV
148
I
LOAD
= 50mA to 1A
146
– 40 –15
1435 G14
85
10
35
60
TEMPERATURE (°C)
4
U W
110
1435 G10
INTV
CC
Regulation
vs INTV
CC
Load Current
V
EXTVCC
= 0V
EXTV
CC
Switch Drop
vs INTV
CC
Load Current
200
180
160
140
120
100
80
60
40
20
25°C
– 55°C
70°C
80
SHUTDOWN CURRENT (µA)
0.3
70°C
0
25°C
– 0.3
60
40
20
0
– 0.5
0
0
10
15
5
INTV
CC
LOAD CURRENT (mA)
20
1435 G08
0
2
4 6 8 10 12 14 16 18 20
INTV
CC
LOAD CURRENT (mA)
1435 G09
RUN/SS Pin Current
vs Temperature
0
– 0.25
3
– 0.50
– 0.75
–1.00
–1.25
0
– 40 –15
SFB Pin Current vs Temperature
2
1
135
85
10
35
60
TEMPERATURE (°C)
110
135
–1.50
– 40 –15
60
35
85
10
TEMPERATURE (°C)
110
135
1435 G11
1435 G12
Transient Response
Transient Response
I
LOAD
= 1A to 3A
1435 G15
110
135
1435 G13
LTC1435
TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Operation
Soft Start: Load Current vs Time
V
OUT
20mV/DIV
V
ITH
200mV/DIV
I
LOAD
= 50mA
PIN FUNCTIONS
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/µF. 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-
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 2.2µF 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.
U W
RUN/SS
5V/DIV
INDUCTOR
CURRENT
1A/DIV
1435 G16
1435 G17
U
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