LT1375_15 Datasheet, PDF (8/28 Pages) Linear Technology – 1.5A, 500kHz Step-Down Switching Regulators

LT1375/LT1376
BLOCK DIAGRAM
connected to an external voltage higher than 3V, bias
power will be drawn from the external source (typically the
regulated output voltage). This will improve efficiency if
the BIAS pin voltage is lower than regulator input voltage.
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-
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.
INPUT
BIAS
2.9V BIAS
REGULATOR
INTERNAL
V_CC
0.05Ω
+–
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 10
SLOPE COMP
SYNC
500kHz
OSCILLATOR
SHUTDOWN
COMPARATOR
+–
0.37V
SHDN
3.5µA
+
–
2.38V
LOCKOUT
COMPARATOR
Σ
0.9V
S
R_S
FLIP-FLOP
+
R
–
CURRENT
COMPARATOR
FOLDBACK _Q2
CURRENT
LIMIT
CLAMP
FREQUENCY
SHIFT CIRCUIT
ERROR
AMPLIFIER
V_C
g_m = 2000µMho
Figure 1. Block Diagram
BOOST
DRIVER
CIRCUITRY
Q1
POWER
SWITCH
V_SW
2.42V
FB
GND
1375/76 BD
APPLICATIONS INFORMATION
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
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.
13756fd