LT1534C View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1534C' PDF : 16 Pages View PDF
LT1534/LT1534-1
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OPERATIO
The negative feedback amplifier allows for direct regula-
tion 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
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 LT1534. 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.
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 LT1534 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.
It is beyond the scope of this data sheet to get into EMI
fundamentals. AN70 contains much information concern-
ing noise in switching regulators and should be consulted.
Oscillator Frequency
The oscillator determines the switching frequency and
therefore the fundamental positioning of all harmonics.
The use of good quality external components is important
to ensure oscillator frequency stability. The oscillator is a
sawtooth design. A current defined by external resistor RT
is used to charge and discharge the capacitor CT. The
discharge rate is approximately ten times the charge rate.
By allowing the user to have control over both compo-
nents, trimming of oscillator frequency can be more easily
achieved.
The external capacitance CT is chosen by:
CT(nF) = 2180/[fOSC(kHz) • RT(kΩ)]
where fOSC is the desired oscillator frequency in kHz.
For RT equal to 16.9k, this simplifies to:
CT(nF) = 129/fOSC(kHz)
(e.g., CT = 1.29nF for fOSC = 100kHz)
A good quality temperature stable capacitor should be
chosen.
Nominally RT should be 16.9k. Since it sets up current, its
temperature coefficient should be selected to compliment
the capacitor. Ideally, both should have low temperature
coefficients.
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