LT1576 Linear Technology, LT1576 Datasheet - Page 8

• Manufacturer Part Number: LT1576
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
LT1576/LT1576-5
BLOCK
FEEDBACK PIN FUNCTIONS
The feedback (FB) pin on the LT1576 is used to set output
voltage and 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 LT1576-5 has internal divider resis-
tors and the FB pin is renamed SENSE, connected directly
to the output.
8
INPUT
SHDN
SYNC
BIAS
COMPARATOR
SHUTDOWN
DIAGRAM
REGULATOR
+
2.9V BIAS
2.44V
3.5 A
U
+
–
0.4V
–
INFORMATION
U
LOCKOUT
COMPARATOR
W
SLOPE COMP
OSCILLATOR
INTERNAL
V
CC
200kHz
W
+
0.025
U
Figure 1. Block Diagram
V
0.8V
FOLDBACK
–
C
CURRENT
+
–
CLAMP
LIMIT
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 35
CURRENT
COMPARATOR
Q2
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.
Please read the following if divider resistors are increased
above the suggested values.
R
1
S
R
g
SHIFT CIRCUIT
R V
m
FREQUENCY
2
FLIP-FLOP
= 1000 Mho
AMPLIFIER
R
S
ERROR
OUT
1 21
.
1 21
.
–
+
CIRCUITRY
DRIVER
1.21V
BOOST
Q1
POWER
SWITCH
1576 BD
GND
V
FB
SW