LT1533C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1533C' PDF : 20 Pages View PDF
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OPERATIO
Output regulation is obtained using the error amp to set
the switch current trip point. The error amp is a transcon-
ductance amplifier that integrates the difference between
the feedback output voltage and an internal 1.25V refer-
ence. The output of the error amp adjusts the switch
current trip point to provide the required load current at
the desired regulated output voltage. This method of
controlling current rather than voltage provides faster
input transient response, cycle by cycle current limiting
for better output switch protection and greater ease in
compensating the feedback loop.
The VC pin serves three different purposes. It is used for
loop compensation, current limit adjustment and soft
starting. During normal operation the VC voltage will be
between 0.2V and 1.33V. An external clamp may be used
for lowering the current limit. A capacitor coupled to an
external clamp can be used for soft starting.
The negative voltage feedback amplifier allows for direct
regulation of negative output voltages. The voltage on the
NFB pin gets amplified by a gain of – 0.5 and driven onto
the FB input, i.e., the NFB pin regulates to – 2.5V while the
amplifier output internally drives the FB pin to 1.25V as in
normal operation. The negative feedback amplifier input
impedance is 100k (typ) referred to ground.
Slew Control
Control of output voltage and current slew rates is done 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 currents generated by these two slewing
events to currents created by external resistors RVSL and
LT1533
RCSL. The two control loops are combined internally to
provide a smooth transition from current slew control to
voltage slew control.
Internal Regulator
Most of the control circuitry operates from an internal 2.4V
low dropout regulator that is powered from VIN. The
internal low dropout design allows VIN to vary from 2.7V
to 23V with virtually no change in device performance.
When the part is put into shutdown, the internal regulator
is turned off, leaving only a small (12µA typ) current drain
from VIN.
Protection Features
There are three modes of protection in the LT1533. The
first is overcurrent limit. This is achieved via the clamping
action of the VC pin. The second is thermal shutdown that
disables both output drivers and pulls the VC pin low in the
event of excessive chip temperature. The third is under-
voltage lockout that also disables both outputs
and pulls the VC pin low whenever VIN drops below 2.5V.
50% Duty Cycle Mode
Since the LT1533 has dual out-of-phase outputs, it is ideal
for driving push-pull transformers. For simple DC trans-
former applications, the part can be forced into a 50% duty
cycle mode using the DUTY pin. Grounding the DUTY pin
will override the internal control circuitry and force the
outputs to switch with a 50% duty cycle at one-half the
oscillator frequency. Slew control also applies in the 50%
duty cycle mode.
APPLICATIONS INFORMATION
Reducing EMI from switching power supplies has tradi-
tionally invoked fear in designers. Many switchers are
designed solely on efficiency and as such produce wave-
forms filled with high frequency harmonics that then
propagate through the rest of the power supply.
The LT1533 provides control over two of the more impor-
tant variables for controlling EMI with switching inductive
loads: switch voltage slew rate and switch current slew
rate. The use of this part will reduce noise and EMI over
conventional switch mode controllers. Because these
variables are under control, a supply built with this part will
exhibit far less tendency to create EMI and less chance of
wandering into problems during production.
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