LTC1435A
High Efficiency Low Noise
Synchronous Step-Down
Switching Regulator
DESCRIPTION
FEATURES
n
Dual N-Channel MOSFET Synchronous Drive The LTC 1435A is a synchronous step-down switching
n
Programmable Fixed Frequency regulator controller that drives external N-channel power
n
Wide V Range: 3.5V to 36V Operation MOSFETs using a fixed frequency architecture. A wide
IN
n
Low Minimum On-Time (300ns) for High duty cycle range of 5% to 99% allows high V to low V
IN OUT
Frequency, Low Duty Cycle Applications DC/DC conversion, as well as low dropout operation that
n
Very Low Dropout Operation: 99% Duty Cycle extends operating time in battery-operated systems. Burst
TM
n
Low Standby Current Mode operation provides high efficiency at low load
n
Secondary Feedback Control currents.
n
Programmable Soft Start
The operating frequency is set by an external capacitor
n
Remote Output Voltage Sense
allowing maximum flexibility in optimizing efficiency. A
n
Logic Controlled Micropower Shutdown: I < 25A
Q
secondary winding feedback control pin, SFB, guarantees
n
Foldback Current Limiting (Optional)
regulation regardless of load on the main output by
n
Current Mode Operation for Excellent Line and Load
forcing continuous operation. Burst Mode operation is
Transient Response
inhibited when the SFB pin is pulled low, which reduces
n
Output Voltages from 1.19V to 9V
noise and RF interference.
n
Available in 16-Lead Narrow SO and SSOP Packages
Soft start is provided by an external capacitor that can be
U
used to properly sequence supplies. The operating cur-
APPLICATIONS
rent level is user-programmable via an external current
n
Notebook and Palmtop Computers, PDAs
sense resistor. Wide input supply range allows operation
n
Cellular Telephones and Wireless Modems
from 3.5V to 30V (36V maximum).
n
Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
n
Burst Mode is a trademark of Linear Technology Corporation.
Battery-Operated Devices
n
DC Power Distribution Systems
TYPICAL APPLICATION
V
IN
4.5V TO 22V
C V
OSC IN C
IN
+
C
OSC
22F
M1
RUN/SS TG
43pF
35V
Si4412DY
C
R
SS
SENSE
2
0.1F
0.033
V
OUT
I SW
TH
1.6V/3A
L1
C
D
C B
LTC1435A 4.7H
330pF CMDSH-3
R1
R
C C
INTV B
OUT
C 35.7k
CC
0.1F 100F
10k
6.3V
SGND
BOOST
R2
2
+
D1
102k
100pF
4.7F
MBRS140T3
M2
V BG
OSENSE
Si4412DY
PGND
+
SENSE SENSE
1000pF
1435A F01
Figure 1. High Efficiency Step-Down Converter
1
+
U
ULTC1435A
ABSOLUTE MAXIMUM RATINGS
PACKAGE/ORDER INFORMATION
Input Supply Voltage (V ).........................36V to 0.3V
IN
TOP VIEW
ORDER PART
Topside Driver Supply Voltage (BOOST)....42V to 0.3V
NUMBER
1
16
C
TG
OSC
Switch Voltage (SW)............................. V + 5V to 5V
IN
2
15
RUN/SS
BOOST
EXTV Voltage ........................................ 10V to 0.3V
CC
LTC1435ACG
3
14
I
SW
TH
+
SENSE , SENSE Voltages ...... INTV + 0.3V to 0.3V
CC
LTC1435ACS
4
13
SFB
V
IN
I , V Voltages .............................. 2.7V to 0.3V
TH OSENSE LTC1435AIG
5
12
SGND
INTV
CC
SFB, Run/SS Voltages .............................. 10V to 0.3V
LTC1435AIS
6
11
V
BG
OSENSE
Peak Driver Output Current < 10s (TG, BG) ............. 2A
7
10
SENSE
PGND
+
INTV Output Current ........................................ 50mA 8 9
SENSE EXTV
CC CC
Operating Ambient Temperature Range
G PACKAGE
S PACKAGE
16-LEAD PLASTIC SSOP 16-LEAD PLASTIC SO
LTC1435AC ............................................ 0C to 70C
T = 125C, = 130C/ W (G)
LTC1435AI ......................................... 40C to 85C JMAX JA
T = 125C, = 110C/ W (S)
JMAX JA
Junction Temperature (Note 1)............................. 125C
Storage Temperature Range ................. 65C to 150C
Consult factory for Military grade parts.
Lead Temperature (Soldering, 10 sec).................. 300C
ELECTRICAL CHARACTERISTICS T = 25C, V = 15V, V = 5V unless otherwise noted.
A IN RUN/SS
SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Main Control Loop
I V Feedback Current (Note 2) 10 50 nA
IN OSENSE
V Feedback Voltage (Note 2) l 1.178 1.19 1.202 V
OSENSE
V Reference Voltage Line Regulation V = 3.6V to 20V (Note 2) 0.002 0.01 %/V
LINEREG IN
V Output Voltage Load Regulation I Sinking 5A (Note 2) l 0.5 0.8 %
LOADREG TH
I Sourcing 5A l 0.5 0.8 %
TH
V Secondary Feedback Threshold V Ramping Negative l 1.16 1.19 1.22 V
SFB SFB
I Secondary Feedback Current V = 1.5V 1 2 A
SFB SFB
V Output Overvoltage Lockout 1.24 1.28 1.32 V
OVL
I Input DC Supply Current EXTV = 5V (Note 3)
Q CC
Normal Mode 3.6V < V < 30V 280 A
IN
Shutdown V = 0V, 3.6V < V < 15V 16 25 A
RUN/SS IN
V Run Pin Threshold l 0.8 1.3 2 V
RUN/SS
I Soft Start Current Source V = 0V 1.5 3 4.5 A
RUN/SS RUN/SS
V Maximum Current Sense Threshold V = 0V, 5V 130 150 180 mV
SENSE(MAX) OSENSE
t Minimum On-Time Tested with Square Wave, SENSE = 1.6V, 250 300 ns
ON(MIN)
V = 20mV (Note 5 )
SENSE
TG Transition Time
TG t Rise Time C = 3000pF 50 150 ns
r LOAD
TG t Fall Time C = 3000pF 50 150 ns
f LOAD
BG Transition Time
BG t Rise Time C = 3000pF 50 150 ns
r LOAD
BG t Fall Time C = 3000pF 40 150 ns
f LOAD
Internal V Regulator
CC
V Internal V Voltage 6V < V < 30V, V = 4V l 4.8 5.0 5.2 V
INTVCC CC IN EXTVCC
V INT INTV Load Regulation I = 15mA, V = 4V 0.2 1 %
LDO CC INTVCC EXTVCC
V EXT EXTV Voltage Drop I = 15mA, V = 5V 130 230 mV
LDO CC INTVCC EXTVCC
V EXTV Switchover Voltage I = 15mA, V Ramping Positive l 4.5 4.7 V
EXTVCC CC INTVCC EXTVCC
2
U W U W
U WW