Datasheet
LT1976/LT1976B
20
1976bfg
APPLICATIO S I FOR ATIO
WUUU
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.
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
IN
to V
OUT
ratios refer to Figure 8 and
the following:
VR
V
R
V
R
IV
V
VR
R
UVLO
SHDN SHDN
SHDN SHDN
HYST
OUT
=++
⎛
⎝
⎜
⎞
⎠
⎟
+
=
()
1
32
1
3
R1 should be chosen to minimize quiescent current during
normal operation by the following equation:
R
VV
I
IN
SHDN MAX
1
2
15
=
()
()
–
.
()
Example:
R
A
M
R
M
M
A
M
k
1
12 2
155
13
3
513
1
65
1
13
649
408
=
μ
()
=Ω
=
Ω
()
=Ω Ω
Ω
μ
Ω
=
–
.
.
.
.
––
.
.
(Nearest 1% 6.49M )
R2 =
1.3
7 – 1.3
1.3M
(Nearest 1% 412k)
BOOST
LT1976
V
BOOST
– V
SW
= V
OUT
V
BOOST(MAX)
= V
IN
+ V
OUT
V
IN
V
OUT
OPTIONAL
(7a)
V
IN
SWGND
BOOST
LT1976
V
BOOST
– V
SW
= V
DC
V
BOOST(MAX)
= V
DC
+ V
IN
V
IN
V
DC
D
SS
1976 F07
V
OUT
(7c)
V
IN
SWGND
BOOST
LT1976
V
BOOST
– V
SW
= V
IN
V
BOOST(MAX)
= 2V
IN
V
IN
V
OUT
(7b)
V
IN
SWGND
Figure 7. BOOST Pin Configurations