Datasheet

LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
7
1765fd
BLOCK DIAGRAM
+
+
INPUT
2.5V BIAS
REGULATOR
1.25MHz
OSCILLATOR
V
SW
FB
V
C
GND
1765 F01
SLOPE COMP
0.005Ω
INTERNAL
V
CC
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 40
SYNC
SHDN
SHUTDOWN
COMPARATOR
CURRENT
COMPARATOR
ERROR
AMPLIFIER
g
m
= 850μMho
BOOST
R
S
FLIP-FLOP
DRIVER
CIRCUITRY
S
R
0.4V
Q1
POWER
SWITCH
PARASITIC DIODES
DO NOT FORWARD BIAS
1.2V
+
+
1.33V
3μA
7μA
INTERNAL
V
CC
The LT1765 is a constant frequency, current mode buck
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 amplifi er, there is a
current sense amplifi er that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscillator
pulse which sets the R
S
ip-fl op to turn the switch on. When
switch current reaches a level set by the inverting input of
the comparator, the fl ip-fl op is reset and the switch turns
off. Output voltage control is obtained by using the output
of the error amplifi er to set the switch current trip point.
This technique means that the error amplifi er 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.
High switch effi ciency is attained by using the BOOST pin
to provide a voltage to the switch driver which is higher
than the input voltage, allowing the switch to be saturated.
This boosted voltage is generated with an external capacitor
and diode. A comparator connected to the shutdown pin
disables the internal regulator, reducing supply current.
Figure 1. Block Diagram