Datasheet

6
LT1961
1961fa
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
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
A comparator connected to the shutdown pin disables the
internal regulator, reducing supply current.
The LT1961 is a constant frequency, current-mode boost
converter. This means that there is an internal clock and
two feedback loops that control the duty cycle of the power
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 R
S
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
Figure 1. Block Diagram
BLOCK DIAGRAM
W
–
+
–
+
Σ
V
IN
2.5V BIAS
REGULATOR
1.25MHz
OSCILLATOR
SW
FB
V
C
GND
GND
1767 F01
SLOPE COMP
0.01Ω
INTERNAL
V
CC
CURRENT SENSE
AMPLIFIER VOLTAGE
GAIN = 40
SYNC
SHDN
SHUTDOWN
COMPARATOR
CURRENT
COMPARATOR
ERROR
AMPLIFIER
g
m
= 850μMho
R
S
FLIP-FLOP
DRIVER
CIRCUITRY
S
R
0.3V
Q1
POWER
SWITCH
1.2V
–
+
–
+
1.35V
3μA
7μA
1
8
5
7
6
3
4
2