LT1961
W
BLOCK DIAGRAM
The LT1961 is a constant frequency, current-mode boost
converter. This means that there is an internal clock and
twofeedbackloopsthatcontrolthedutycycleofthepower
switch. In addition to the normal error amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscilla-
tor pulse which sets the RS flip-flop to turn the switch on.
When switch current reaches a level set by the inverting
input of the comparator, the flip-flop is reset and the
switch turns off. Output voltage control is obtained by
using the output of the error amplifier to set the switch
current trip point. This technique means that the error
amplifier commands current to be delivered to the output
rather than voltage. A voltage fed system will have low
phase shift up to the resonant frequency of the inductor
and output capacitor, then an abrupt 180° shift will occur.
The current fed system will have 90° phase shift at a much
lower frequency, but will not have the additional 90° shift
until well beyond the LC resonant frequency. This makes
itmucheasiertofrequencycompensatethefeedbackloop
and also gives much quicker transient response.
A comparator connected to the shutdown pin disables the
internal regulator, reducing supply current.
V
IN
1
INTERNAL
CC
2.5V BIAS
REGULATOR
V
SLOPE COMP
Σ
0.3V
SW
2
1.25MHz
S
R
8
SYNC
Q1
POWER
SWITCH
OSCILLATOR
R
DRIVER
CURRENT
COMPARATOR
S
FLIP-FLOP
CIRCUITRY
+
–
CURRENT SENSE
SHUTDOWN
COMPARATOR
AMPLIFIER VOLTAGE
GAIN = 40
7μA
–
+
+
–
0.01Ω
1.35V
–
SHDN
5
6
FB
3μA
+
ERROR
3
4
GND
V
C
7
AMPLIFIER
1.2V
g
m
= 850μMho
GND
1767 F01
Figure 1. Block Diagram
1961fa
6
Linear [ Linear ]
