LT3430 Linear Technology, LT3430 Datasheet - Page 7

• Manufacturer Part Number: LT3430
• Description: High Voltage/ 3A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

BLOCK DIAGRA
The LT3430 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
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
SHDN
SYNC
BIAS
V
IN
COMPARATOR
3, 4
10
14
15
SHUTDOWN
REGULATOR
+
2.9V BIAS
2.38V
S
W
0.4V
flip-flop to turn the switch on.
–
+
–
COMPARATOR
LOCKOUT
5.5 A
INTERNAL
V
CC
ANTISLOPE COMP
SLOPE COMP
OSCILLATOR
200kHz
Figure 1. LT3430 Block Diagram
V
CLAMP
C(MAX)
Q3
+
it much easier to frequency compensate the feedback loop
and also gives much quicker transient response.
Most of the circuitry of the LT3430 operates from an
internal 2.9V bias line. The bias regulator normally draws
power from the regulator input pin, but if the BIAS pin is
connected to an external voltage equal to or higher than
3V, bias power will be drawn from the external source
(typically the regulated output voltage). This will improve
efficiency if the BIAS pin voltage is lower than regulator
input voltage.
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
capacitor and diode. Two comparators are connected to
the shutdown pin. One has a 2.38V threshold for under-
voltage lockout and the second has a 0.4V threshold for
complete shutdown.
11
V
R
C
1
LIMIT
FOLDBACK
CURRENT
–
CLAMP
LIMIT
CURRENT
COMPARATOR
Q2
S
R
FLIP-FLOP
R
g
m
S
FREQUENCY
= 2000 Mho
FOLDBACK
AMPLIFIER
ERROR
BOOST
CIRCUITRY
6
DRIVER
–
+
1.22V
1766 F01
2, 5
12
Q1
POWER
SWITCH
R
SW
FB
GND
1, 8, 9, 16
SENSE
LT3430
sn3430 3430is
7