LT1578 Linear Technology, LT1578 Datasheet - Page 7

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

BLOCK
The LT1578 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
INPUT
SHDN
SYNC
COMPARATOR
SHUTDOWN
DIAGRAM
REGULATOR
+
2.9V BIAS
2.42V
3.5 A
–
+
0.4V
–
LOCKOUT
COMPARATOR
S
W
flip-flop to turn the switch on.
SLOPE COMP
OSCILLATOR
INTERNAL
V
CC
200kHz
+
0.025
Figure 1. Block Diagram
V
0.8V
–
FOLDBACK
C
CURRENT
+
–
CLAMP
LIMIT
CURRENT SENSE
AMPLIFIER DC
VOLTAGE GAIN = 35
CURRENT
COMPARATOR
Q2
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 saturate. This
boosted voltage is generated with an external capacitor
and diode. Two comparators are connected to the shut-
down pin. One has a 2.42V threshold for undervoltage
lockout and the second has a 0.4V threshold for complete
shutdown.
S
R
g
SHIFT CIRCUIT
m
FREQUENCY
FLIP-FLOP
= 1000 Mho
AMPLIFIER
R
S
ERROR
LT1578/LT1578-2.5
–
+
CIRCUITRY
DRIVER
1.21V
BOOST
Q1
POWER
SWITCH
1578 BD
V
FB
GND
7
SW