3439EFE データシートの表示（PDF） - Linear Technology
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3439EFE Datasheet PDF : 12 Pages
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BLOCK DIAGRA
VIN
11 13
SHDN VIN
LDO
REGULATOR
INTERNAL VCC
RT
RT
7
CT
CT
6
OSCILLATOR
SYNC
5
LT3439
Q
T FF
QB
SLEW
CONTROL
GND
RSL
10
4
RSL
LT3439
D1
T1
••
••
VOUT
COUT
D2
3
COLA
14
COLB
OUTPUT
DRIVERS
RSENSE
16
PGND 1
3439 BD
U
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, VIN is applied across the primary side of
the transformer. The voltage on the secondary side of the
transformer is simply VIN times the turns ratio. The diodes
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 VIN
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
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 RSL. The two
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 VIN. VIN can
sn3439 3439fs
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