5
LT1776
TYPICAL PERFORMANCE CHARACTERISTICS
UW
V
C
Pin Switching Threshold,
Boost Threshold, Clamp Voltage
vs Temperature
TEMPERATURE (°C)
–50
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
25 75
LT1776 G10
–25 0
50 100 125
V
C
PIN VOLTAGE (V)
CLAMP
VOLTAGE
BOOST
THRESHOLD
SWITCHING
THRESHOLD
TEMPERATURE (°C)
–50
750
700
650
600
550
500
450
400
25 75
LT1776 G12
–25 0
50 100 125
TRANSCONDUCTANCE (µmho)
Error Amplifier Transconductance
vs Temperature
FB PIN VOLTAGE (V)
1.0
FEEDBACK AMPLIFIER OUTPUT CURRENT (µA)
100
50
0
–50
–100
–150
1.4
1776 G11
1.1
1.2
1.3
1.5
25°C
–55°C
125°C
Feedback Amplifier Output
Current vs FB Pin Voltage
PIN FUNCTIONS
UUU
SHDN (Pin 1): When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regulator
and reduces V
IN
input current to a few tens of microam-
peres (shutdown mode).
When this pin is held above the shutdown mode threshold,
but below the lockout threshold, the part will be opera-
tional with the exception that output switching action will
be inhibited (lockout mode). A user-adjustable undervolt-
age lockout can be implemented by driving this pin from
an external resistor divider to V
IN
. This action is logically
“ANDed” with the internal UVLO, set at nominally 6.7V,
such that minimum V
IN
can be increased above 6.7V, but
not decreased (see Applications Information).
If unused, this pin should be left open. However, the high
impedance nature of this pin renders it susceptible to
coupling from the high speed V
SW
node, so a small
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open”.
V
CC
(Pin 2): This pin is used to power the internal control
circuitry off of the switching supply output. Proper use of
this pin enhances overall power supply efficiency. During
start-up conditions, internal control circuitry is powered
directly from V
IN
.
If the output capacitor is located more than one inch from
the V
CC
pin, a separate 0.1µF bypass capacitor to ground
may be required right at the pin.
V
SW
(Pin 3): This is the emitter node of the output switch
and has large currents flowing through it. This node
moves at a high dV/dt rate, especially when in “boost”
mode. Keep the traces to the switching components as
short as possible to minimize electromagnetic radiation
and voltage spikes.
GND (Pin 4): This is the device ground pin. The internal
reference and feedback amplifier are referred to it. Keep
the ground path connection to the FB divider and the V
C
compensation capacitor free of large ground currents.
V
IN
(Pin 5): This is the high voltage supply pin for the
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the V
CC
pin is left
open. A high quality bypass capacitor which meets the
input ripple current requirements is needed here. (See
Applications Information).
SYNC (Pin 6): Pin used to synchronize internal oscillator
to the external frequency reference. It is directly logic
compatible and can be driven with any signal between
