lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 10

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

LT1976/LT1976B
BLOCK DIAGRA
The LT1976 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
10
10
14
15
12
11
13
4
9
7
V
BIAS
SYNC
SHDN
C
FB
V
PGFB
C
IN
SS
C
T
1.25V
1.12V
1.3V
+
–
COMP
SHDN
W
UNDERVOLTAGE
–
+
+
–
INTERNAL REF
ERROR
SOFT-START
COMP
SHUTDOWN
AMP
FOLDBACK
LOCKOUT
THERMAL
PG
DETECT
Figure 1. LT1976/LT1976B Block Diagram
2.4V
BURST MODE
LT1976 ONLY
OSCILLATOR
ANTISLOPE
DETECT
1.2V C
200kHz
CLAMP
CLAMP
SLOPE
COMP
COMP
V
C
T
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 LT1976 operates from an
internal 2.4V bias line. The bias regulator normally draws
Σ
CURRENT
COMP
R
S
SWITCH
+
–
LATCH
Q
CIRCUITRY
DRIVER
BOOST
PGND
GND
SW
PG
1976 BD
1976bfg
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