lt1776 Linear Technology Corporation, lt1776 Datasheet - Page 5

• Manufacturer Part Number: lt1776
• Description: Wide Input Range, High Efficiency, Step-down Switching Regulator
• Manufacturer: Linear Technology Corporation

PIN
SHDN (Pin 1): When pulled below the shutdown mode
threshold, nominally 0.30V, this pin turns off the regulator
and reduces V
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
“ANDed” with the internal UVLO, set at nominally 6.7V,
such that minimum V
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
capacitor to ground, typically 100pF or so is recom-
mended when the pin is left “open”.
V
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
TYPICAL PERFORMANCE CHARACTERISTICS
CC
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
U
(Pin 2): This pin is used to power the internal control
–50
V
Boost Threshold, Clamp Voltage
vs Temperature
C
FUNCTIONS
Pin Switching Threshold,
–25
U
0
TEMPERATURE ( C)
IN
IN
.
input current to a few tens of microam-
25
U
50
IN
THRESHOLD
THRESHOLD
SWITCHING
VOLTAGE
can be increased above 6.7V, but
CLAMP
BOOST
75
W
100
LT1776 G10
IN
. This action is logically
U
125
SW
node, so a small
–100
–150
–50
100
50
0
1.0
Feedback Amplifier Output
Current vs FB Pin Voltage
1.1
FB PIN VOLTAGE (V)
1.2
If the output capacitor is located more than one inch from
the V
may be required right at the pin.
V
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
compensation capacitor free of large ground currents.
V
output switch. It also supplies power to the internal control
circuitry during start-up conditions or if the V
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
SW
IN
1.3
(Pin 5): This is the high voltage supply pin for the
(Pin 3): This is the emitter node of the output switch
CC
pin, a separate 0.1 F bypass capacitor to ground
1.4
25 C
–55 C
125 C
1776 G11
1.5
750
700
650
600
550
500
450
400
–50
Error Amplifier Transconductance
vs Temperature
–25
0
TEMPERATURE ( C)
25
50
LT1776
CC
75
pin is left
100
LT1776 G12
5
125
C