LT3439 LINER [Linear Technology], LT3439 Datasheet - Page 5

• Manufacturer Part Number: LT3439
• Description: Slew Rate Controlled Ultralow Noise1A Isolated DC/DC Transformer Driver
• Manufacturer: LINER [Linear Technology]

BLOCK DIAGRA
OPERATIO
Push-Pull Topology
The push-pull DC transformer topology is a very straight-
forward switching power supply. The two switches are
turned on out of phase at 50% duty cycles. During the
switch on time, V
the transformer. The voltage on the secondary side of the
transformer is simply V
rectify the secondary voltage and generate the output
voltage. The output capacitor is for hold-up and filtering.
Some of the topology’s advantages are: 1) Stepping up or
down the input voltage can easily be done by setting the
turns ratio. 2) The transformer provides isolation between
the input and output. 3) Each switch cycle applies V
across the transformer in opposite polarities. Therefore,
the transformer core never saturates and a separate reset
circuit is not necessary.
The push-pull topology is not without its concerns. An
imbalance in the two sides of the transformer can
R
C
T
T
7
6
5
U
IN
R
C
SYNC
T
is applied across the primary side of
T
IN
INTERNAL V
OSCILLATOR
REGULATOR
11
times the turns ratio. The diodes
W
SHDN V
LDO
V
13
IN
CC
IN
T
FF
QB
Q
10
GND
LT3439
CONTROL
4
SLEW
R
IN
SL
R
SL
eventually cause the transformer to saturate. Also, dur-
ing the turn-off of the switches, the leakage inductance
causes a large undesirable voltage spike. The LT3439
slew control feature addresses both of these concerns
and is discussed in the Applications Information section.
Slew Control
Control of voltage and current slew rate is maintained via
two feedback loops. One loop controls the output switch
collector voltage dV/dt and the other loop controls the
emitter current dI/dt. Output slew control is achieved by
comparing the two currents generated by these slewing
events to a current set by the external resistor R
control loops work together to provide a smooth transition
from voltage slew control to current slew control.
Internal Regulator
Most of the control circuitry operates from an internal 2.4V
low dropout regulator that is powered from V
+
–
COLA
3
OUTPUT
DRIVERS
R
SENSE
COLB
PGND
14
3439 BD
•
•
16
T1
1
•
•
D1
D2
V
OUT
LT3439
C
IN
SL
OUT
. V
. The two
sn3439 3439fs
IN
5
can