LT1375 Linear Technology, LT1375 Datasheet - Page 8

• Manufacturer Part Number: LT1375
• Description: 1.5A/ 500kHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

LT1375/LT1376
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing switch to be saturated.
This boosted voltage is generated with an external capaci-
APPLICATIONS
BLOCK
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1376 is used to set output
voltage and also to provide several overload protection
features. The first part of this section deals with selecting
resistors to set output voltage and the remaining part talks
about foldback frequency and current limiting created by
the FB pin. Please read both parts before committing to a
final design. The fixed 5V LT1376-5 has internal divider
8
INPUT
SHDN
SYNC
BIAS
COMPARATOR
SHUTDOWN
DIAGRAM
REGULATOR
+
U
2.9V BIAS
2.38V
3.5 A
+
–
0.4V
–
INFORMATION
U
W
LOCKOUT
COMPARATOR
OSCILLATOR
SLOPE COMP
INTERNAL
V
CC
500kHz
W
+
U
0.1
Figure 1. Block Diagram
FOLDBACK
V
0.9V
–
CURRENT
C
+
–
CLAMP
LIMIT
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 10
CURRENT
COMPARATOR
Q2
tor and diode. Two comparators are connected to the
shutdown pin. One has a 2.38V threshold for undervoltage
lockout and the second has a 0.4V threshold for complete
shutdown.
resistors and the FB pin is renamed SENSE, connected
directly to the output.
The suggested value for the output divider resistor (see
Figure 2) from FB to ground (R2) is 5k or less, and a
formula for R1 is shown below. The output voltage error
caused by ignoring the input bias current on the FB pin is
less than 0.25% with R2 = 5k. A table of standard 1%
values is shown in Table 1 for common output voltages.
S
R
g
SHIFT CIRCUIT
m
FREQUENCY
FLIP-FLOP
= 2000 Mho
AMPLIFIER
R
S
ERROR
+
–
CIRCUITRY
DRIVER
2.42V
BOOST
Q1
POWER
SWITCH
1375/76 BD
V
FB
GND
SW