LT1375/LT1376
W
BLOCK DIAGRAM
connected to an external voltage higher than 3V, bias
powerwillbedrawnfromtheexternalsource(typicallythe
regulated output voltage). This will improve efficiency if
the BIAS pin voltage is lower than regulator input voltage.
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
shutdownpin. Onehasa2.38Vthresholdforundervoltage
lockout and the second has a 0.4V threshold for complete
shutdown.
High switch efficiency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
0.05Ω
INPUT
+
–
CURRENT
SENSE
AMPLIFIER
2.9V BIAS
REGULATOR
INTERNAL
CC
BIAS
V
VOLTAGE GAIN = 10
SLOPE COMP
BOOST
Σ
0.9V
500kHz
OSCILLATOR
S
R
SYNC
Q1
POWER
SWITCH
R
DRIVER
CIRCUITRY
S
FLIP-FLOP
+
–
SHUTDOWN
COMPARATOR
V
SW
+
–
CURRENT
COMPARATOR
0.37V
FREQUENCY
SHIFT CIRCUIT
SHDN
3.5µA
FOLDBACK
CURRENT
LIMIT
Q2
+
–
CLAMP
–
FB
LOCKOUT
COMPARATOR
+
ERROR
AMPLIFIER
2.38V
g
m
= 2000µMho
2.42V
V
C
GND
1375/76 BD
Figure 1. Block Diagram
U
W U U
APPLICATIONS INFORMATION
FEEDBACK PIN FUNCTIONS
resistors and the FB pin is renamed SENSE, connected
directly to the output.
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
resistorstosetoutputvoltageandtheremainingparttalks
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
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
8
Linear [ Linear ]
