lt3573 Linear Technology Corporation, lt3573 Datasheet - Page 5

• Manufacturer Part Number: lt3573
• Description: Isolated Flyback Converter Without An Opto-coupler
• Manufacturer: Linear Technology Corporation

PIN FUNCTIONS
GND: Ground.
TEST: This pin is used for testing purposes only and must
be connected to ground for the part to operate properly.
SW: Collector Node of the Output Switch. This pin has
large currents fl owing through it. Keep the traces to the
switching components as short as possible to minimize
electromagnetic radiation and voltage spikes.
V
start-up circuitry and as a reference voltage for the DCM
comparator and feedback circuitry. This pin must be locally
bypassed with a capacitor.
BIAS: Bias Voltage. This pin supplies current to the switch
driver and internal circuitry of the LT3573. This pin must
be locally bypassed with a capacitor. This pin may also be
connected to V
≤ 15V. If a third winding is used, the BIAS voltage should
be lower than the input voltage for proper operation.
SHDN/UVLO: Shutdown/Undervoltage Lockout. A resistor
divider connected to V
minimum input voltage at which the LT3573 will operate.
At a voltage below ~0.7V, the part draws no quiescent
current. When below 1.25V and above ~0.7V, the part will
draw 10μA of current, but internal circuitry will remain off.
Above 1.25V, the internal circuitry will start and a 10μA
current will be fed into the SS pin. When this pin falls
below 1.25V, 2.5μA will be pulled from the pin to provide
programmable hysteresis for UVLO.
R
should be tied to this pin to ground to set the current
limit. Use a 10k resistor for the full current capabilities
of the switch.
IN
ILIM
: Input Voltage. This pin supplies current to the internal
: Maximum Current Limit Adjust Pin. A resistor
IN
if a third winding is not used and if V
IN
is tied to this pin to program the
IN
SS: Soft-Start Pin. Place a soft-start capacitor here to
limit start-up inrush current and output voltage ramp
rate. Switching starts when the voltage at this pin reaches
~0.7V.
V
a series RC from this pin to ground to compensate the
switching regulator. A 100pF capacitor in parallel helps
eliminate noise.
R
connected to the transformer primary (V
of this resistor to the R
bandgap reference, determines the output voltage (plus
the effect of any non-unity transformer turns ratio). The
average current through this resistor during the fl yback
period should be approximately 200μA. For nonisolated
applications, this pin should be connected to V
R
Resistor. This resistor should be in the range of 6k, but
for convenience, need not be precisely this value. For
nonisolated applications, a traditional resistor voltage
divider may be connected to this pin.
T
a resistor to ground to produce a current proportional to
absolute temperature to be sourced into the R
I
Exposed Pad: Ground. The Exposed Pad of the package
provides both electrical contact to ground and good thermal
contact to the printed circuit board. The Exposed Pad must
be soldered to the circuit board for proper operation and
should be well connected with many vias to an internal
ground plane.
TC
C
C
FB
REF
: Output Voltage Temperature Compensation. Connect
: Compensation Pin for Internal Error Amplifi er. Connect
= 0.55V/R
: Input Pin for External Feedback Resistor. This pin is
: Input Pin for External Ground-Referred Reference
TC
.
REF
resistor, times the internal
SW
LT3573
). The ratio
IN
REF
.
node.
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3573f