lt1976 Linear Technology Corporation, lt1976 Datasheet - Page 20

• Manufacturer Part Number: lt1976
• Description: High Voltage 1.5a, 200khz Step-down Switching Regulator With 100?a Quiescent Current
• Manufacturer: Linear Technology Corporation

LT1976/LT1976B
APPLICATIO S I FOR ATIO
A 0.33μF boost capacitor is recommended for most appli-
cations. Almost any type of film or ceramic capacitor is
suitable but the ESR should be <1Ω to ensure it can be fully
recharged during the off time of the switch. The capacitor
value is derived from worst-case conditions of 4700ns on
time, 42mA boost current and 0.7V discharge ripple. The
boost capacitor value could be reduced under less de-
manding conditions but this will not improve circuit opera-
tion or efficiency. Under low input voltage and low load
conditions a higher value capacitor and Schottky boost
diode will reduce discharge ripple and improve start-up
and dropout operation.
20
V
V
V
IN
IN
IN
Figure 7. BOOST Pin Configurations
V
V
BOOST(MAX)
BOOST(MAX)
V
V
V
V
BOOST
BOOST
BOOST
BOOST(MAX)
U
V
GND
V
GND
V
GND
IN
IN
IN
LT1976
LT1976
LT1976
– V
– V
– V
BOOST
BOOST
BOOST
SW
= V
SW
= V
SW
U
SW
SW
SW
IN
= 2V
= V
DC
= V
= V
(7a)
(7b)
(7c)
+ V
OUT
+ V
DC
IN
IN
OUT
IN
OPTIONAL
D
W
SS
1976 F07
V
V
V
V
OUT
OUT
DC
OUT
U
SHUTDOWN FUNCTION AND UNDERVOLTAGE
LOCKOUT
The SHDN pin on the LT1976 controls the operation of the
IC. When the voltage on the SHDN pin is below the 1.2V
shutdown threshold the LT1976 is placed in a “zero”
supply current state. Driving the SHDN pin above the
shutdown threshold enables normal operation. The SHDN
pin has an internal sink current of 3μA.
In addition to the shutdown feature, the LT1976 has an
undervoltage lockout function. When the input voltage is
below 2.4V, switching will be disabled. The undervoltage
lockout threshold doesn’t have any hysteresis and is
mainly used to insure that all internal voltages are at the
correct level before switching is enabled. If an undervolt-
age lockout function with hysteresis is needed to limit
input current at low V
the following:
R1 should be chosen to minimize quiescent current during
normal operation by the following equation:
Example:
R
R
R
R2 =
V
V
UVLO
HYST
1
1
3
=
=
=
=
( )
1 5 5
1.3M
408
5 1 3
7 – 1.3
1 5
.
=
12 2
=
(
.
(
R
.
V
k
–
V
(
1
1
OUT
μ
I
M
Ω
IN
(Nearest 1% 412k)
⎛
⎜
⎝
SHDN MAX
A
R
Ω
V
–
)
–
3
( )
SHDN
R
R
)
2
=
1
3
(
μ
1
1.3
V
=
1 3
IN
A
.
6 5
to V
.
–
+
M
)
M
)
6 49
V
Ω
.
OUT
SHDN
Ω
R
1 3
2
.
(Nearest 1% 6.49M )
M
ratios refer to Figure 8 and
Ω
+
I
SHDN
⎞
⎟ +
⎠
V
SHDN
Ω
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