LT3781_15 Datasheet, PDF (6/20 Pages) Linear Technology – Transistor Synchronous Forward Controller

LT3781
TYPICAL PERFOR A CE CHARACTERISTICS
Soft-Start Output Current
vs Soft-Start Pin Voltage
60
T_A = 25°C
40
20
0
0
1
2
3
4
5
SOFT-START PIN VOLTAGE (V)
3781 G13
Current Sense Amplifier
Bandwidth vs Temperature
8
7
6
5
4
3
2
–55 –35 –15 5 25 45 65 85 105 125
TEMPERATURE (°C)
3781 G14
PI FU CTIO S
SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive going threshold of 1.25V enables the LT3781.
150mV of input hysteresis resists mode switching insta-
bility.
The SHDN pin can be controlled by either a logic level input
or with an analog signal. This shutdown feature is typically
used for input supply undervoltage protection. A resistor
divider from the converter input supply to the SHDN pin
monitors that supply for control of system power-up
sequencing, etc.
An 18V clamp on the V_CC pin is enabled during shutdown
mode, preventing a trickle start circuit from pulling that pin
above maximum operational levels. All other internal
functions are disabled during shutdown.
OVLO (Pin 2): Overvoltage Shutdown Sense. Typically
connected to input supply through a resistor divider. If pin
voltage exceeds 1.25V, LT3781 switching function is
disabled to protect boosted circuitry from exceeding ab-
solute maximum voltage. 40mV of input hysteresis resists
mode switching instability. Exceeding the OVLO threshold
also triggers soft-start reset, resulting in a graceful recov-
ery from an input transient event.
6
THERM (Pin 3): System Thermal Shutdown. Auxiliary
shutdown pin that is typically used for system thermal
protection. If pin voltage exceeds 1.25V, LT3781
switching function is disabled. 40mV of input hysteresis
resists mode switching instability. Exceeding the THERM
threshold also triggers soft-start reset, resulting in a
graceful recovery.
SGND (Pin 4): Signal Ground Reference. Careful board
layout techniques must be used to prevent corruption of
signal ground reference. High-current switching paths
must be oriented on the converter ground plane such that
currents to/from the switches do not affect the integrity of
the LT3781 signal ground reference.
5V_REF (Pin 5): 5V Local Reference. Allows connection of
external loads up to 20mA DC. Typically bypassed with
1µF ceramic capacitor to SGND. Reference output is
current limit protected to a typical value of 45mA. If the
load on the 5V reference exceeds the current limit value,
LT3781 switching function is disabled and the soft-start
function is reset.
FSET (Pin 6): Oscillator Timing Pin. Connect a resistor
(R_FSET) from the 5V_REF pin to this pin and a capacitor
(C_FSET) from this pin to ground.
3781f