LT1765/LT1765-1.8/LT1765-2.5/
LT1765-3.3/LT1765-5
BLOCK DIAGRAM
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 amplifier, there is a
current sense amplifier that monitors switch current on a
cycle-by-cycle basis. A switch cycle starts with an oscillator
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 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.
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 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.
0.005Ω
INPUT
+
2.5V BIAS
REGULATOR
INTERNAL
V
CC
–
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 40
SLOPE COMP
∑
0.4V
BOOST
SYNC
1.25MHz
OSCILLATOR
S
CURRENT
COMPARATOR
R
S
FLIP-FLOP
DRIVER
CIRCUITRY
+
SHUTDOWN
COMPARATOR
R
Q1
POWER
SWITCH
–
7μA
PARASITIC DIODES
DO NOT FORWARD BIAS
V
SW
1.33V
ERROR
AMPLIFIER
g
m
= 850μMho
SHDN
3μA
INTERNAL
V
CC
V
C
Figure 1. Block Diagram
–
+
1.2V
–
FB
GND
1765 F01
+
1765fd
7
LINER [ LINEAR TECHNOLOGY ]
