LT1977
BLOCK DIAGRA
4
V
IN
10
BIAS
14
15
SYNC
SHDN
ANTISLOPE
COMP
R
S
+
SHDN
COMP
–
1.3V
C
SS
BURST MODE
DETECT
9
12
FB
1.25V
11
13
V
C
PG
PGFB
16
1.12V
7
C
T
The LT1977 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 RS latch to turn the switch on. When
switch current reaches a level set by the current compara-
tor the latch is reset and the switch turns off. Output volt-
age 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
8
W
INTERNAL REF
UNDERVOLTAGE
LOCKOUT
THERMAL
SHUTDOWN
2.4V
SLOPE
COMP
Σ
500kHz
OSCILLATOR
+
CURRENT
COMP
–
BOOST
6
SWITCH
Q
LATCH
DRIVER
CIRCUITRY
SW
2
SOFT-START
FOLDBACK
DETECT
V
C
CLAMP
+
ERROR
AMP
–
+
PG
COMP
1.2V C
T
CLAMP
GND
17
PGND
8
–
1977 BD
Figure 1. LT1977 Block Diagram
delivered to the output rather than voltage. A voltage fed
system will have low phase shift up to the resonant fre-
quency 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 fre-
quency. This makes it much easier to frequency compen-
sate the feedback loop and also gives much quicker tran-
sient response.
Most of the circuitry of the LT1977 operates from an
internal 2.4V bias line. The bias regulator normally draws
1977f
LINER [ LINEAR TECHNOLOGY ]
