6
LT1681
1681f
TYPICAL PERFOR A CE CHARACTERISTICS
UW
Soft-Start Output Current
vs Soft-Start Pin Voltage
Current Sense Amplifier
Bandwidth vs Temperature
SOFT-START PIN VOLTAGE (V)
0
SOFT-START OUTPUT CURRENT (µA)
20
40
60
10234
1681 G13
5
T
A
= 25°C
TEMPERATURE (°C)
–55
2
CURRENT SENSE AMP BANDWIDTH (MHz)
3
5
6
7
–15
25
45 125
1681 G14
4
–35 5
65
85
105
8
SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive-going threshold of 1.25V enables the LT1681.
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. All internal functions are dis-
abled during shutdown.
OVLO (Pin 2): Overvoltage Shutdown Sense. Typically
connected to input supply through a resistor divider. If pin
voltage exceeds 1.25V, the LT1681 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.
THERM (Pin 3): System Thermal Shutdown. Auxiliary
shutdown pin that is typically used for system thermal
protection. If pin voltage exceeds 1.25V, the LT1681
switching function is disabled. 40mV of input hysteresis
UU
U
PI FU CTIO S
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
the 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 LT1681 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,
LT1681 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.
The LT1681 oscillator operates by monitoring the voltage
on C
FSET
as it is charged via R
FSET
. When the voltage on the
FSET pin reaches 2.5V, the oscillator rapidly discharges
the capacitor with an average current of 0.8mA. Once the
