LT1767 Linear Technology, LT1767 Datasheet - Page 6

• Manufacturer Part Number: LT1767
• Description: Monolithic 1.5A/ 1.25MHz Step-Down Switching Regulators
• Manufacturer: Linear Technology

LT1767/LT1767-1.8/
LT1767-2.5/LT1767-3.3/LT1767-5
BLOCK
The LT1767 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 oscilla-
tor pulse which sets the R
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
6
COMPARATOR
SHDN
SYNC
SHUTDOWN
V
IN
2
8
5
DIAGRAM
REGULATOR
+
2.5V BIAS
1.33V
A
–
S
W
flip-flop to turn the switch on.
SLOPE COMP
OSCILLATOR
7 A
INTERNAL
V
1.25MHz
CC
+
0.01
7
V
0.35V
Figure 1. Block Diagram
–
C
–
+
CURRENT
SENSE
AMPLIFIER
VOLTAGE GAIN = 40
CURRENT
COMPARATOR
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 switch to be saturated.
This boosted voltage is generated with an external capaci-
tor and diode. A comparator connected to the shutdown
pin disables the internal regulator, reducing supply
current.
S
R
g
m
FLIP-FLOP
= 850 Mho
AMPLIFIER
R
S
ERROR
–
+
CIRCUITRY
PARASITIC DIODES
DRIVER
FORWARD BIAS
1.2V
DO NOT
1
BOOST
3
6
4
Q1
POWER
SWITCH
sn1767 1767fas
1767 F01
GND
V
FB
SW