Datasheet

LT3434
8
3434fb
BLOCK DIAGRA
W
12
FB
V
C
Σ
15
INTERNAL REF
UNDERVOLTAGE
LOCKOUT
THERMAL
SHUTDOWN
SOFT-START
FOLDBACK
DETECT
SLOPE
COMP
ANTISLOPE
COMP
1.2V C
T
CLAMP
2.4V
–
+
1.3V
1.25V
SHDN
9
C
SS
14
SYNC
10
BIAS
4
V
IN
SHDN
COMP
–
+
–
+
ERROR
AMP
11
13
PGFB
C
T
1.12V
–
+
PG
COMP
7
BURST MODE
DETECT
200kHz
OSCILLATOR
SWITCH
LATCH
CURRENT
COMP
DRIVER
CIRCUITRY
R
Q
SW
SW
S
2
BOOST
6
PG
16
GND
17
PGND
3434 BD
8
V
C
CLAMP
3
V
IN
5
The LT3434 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
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 LT3434 operates from an
internal 2.4V bias line. The bias regulator normally draws
Figure 1. LT3434 Block Diagram