LT3575
5
3575f
PIN FUNCTIONS
NC (Pins 1, 15, 16): No Connect Pins. Can be left open
or connected to any ground plane.
V
IN
(Pin 2): Input Voltage. This pin supplies current to
the internal start-up circuitry and as a reference voltage
for the DCM comparator and feedback circuitry. This pin
must be locally bypassed with a capacitor.
SW (Pins 3, 4): 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.
BIAS (Pin 5): Bias Voltage. This pin supplies current to the
switch driver and internal circuitry of the LT3575. This pin
must be locally bypassed with a capacitor. This pin may
also be connected to V
IN
if a third winding is not used and if
V
IN
≤ 15V. If a third winding is used, the BIAS voltage should
be lower than the input voltage for proper operation.
SHDN/UVLO (Pin 6): Shutdown/Undervoltage Lockout.
A resistor divider connected to V
IN
is tied to this pin to
program the minimum input voltage at which the LT3575
will operate. At a voltage below ~0.7V, the part draws no
quiescent current. When below 1.22V and above ~0.7V,
the part will draw 7μA of current, but internal circuitry will
remain off. Above 1.22V, the internal circuitry will start
and a 7μA current will be fed into the SS pin. When this
pin falls below 1.22V, 2.8μA will be pulled from the pin to
provide programmable hysteresis for UVLO.
SS (Pin 7): 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.
R
ILIM
(Pin 8): Maximum Current Limit Adjust Pin. A resistor
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.
V
C
(Pin 9): Compensation Pin for Internal Error Amplifi er.
Connect a series RC from this pin to ground to compensate
the switching regulator. A 100pF capacitor in parallel helps
eliminate noise.
R
FB
(Pin 10): Input Pin for External Feedback Resistor. This
pin is connected to the transformer primary (V
SW
). The
ratio of this resistor to the R
REF
resistor, times the internal
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
IN
.
R
REF
(Pin 11): Input Pin for External Ground-Referred
Reference 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.
TC (Pin 12): Output Voltage Temperature Compensation.
Connect a resistor to ground to produce a current
proportional to absolute temperature to be sourced into
the R
REF
node. I
TC
= 0.55V/R
TC
.
TEST (Pin 13): This pin is used for testing purposes only
and must be connected to ground for the part to operate
properly.
GND (Pin 14, Exposed Pad Pin 17): 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.
