13
LT1500/LT1501
APPLICATIONS INFORMATION
WUU
U
and it should be no more than 300k to keep bias current
errors under 1%, giving:
R
RV
V
R
R
V
DIV BAT
BAT
3
124
4
3124
124
=
()
=
()
.
.
–.
V
BAT
= low battery voltage
R
DIV
= Thevenin divider resistance = R3 in parallel with R4
There is about 20mV of hysteresis at the LBI pin. Hyster-
esis can be increased by adding a resistor (R5) from the
output (LBO) back to LBI. This resistor can be calculated
from the following equation, but note that the equation for
R4 will have to be changed when R5 is added.
R
RV
VmVV
CC
HYST BAT
5
3
17
=
()
() ()
–
V
CC
= supply voltage for LBO pull-up resistor
V
HYST
= desired hysteresis at the battery
R4 (When R5 is Used)=
R3 R5
()( )
−
()
+−
()
124
5 1 24 3 1 24
.
..RV RV
BAT CC
The LBO pin is open collector. The external pull-up resistor
value is determined by user needs. Generally the resistor
is 100k to 1M to keep current drain low, but the LBO pin
can sink several milliamperes if needed.
Example: low battery voltage = 2.5V, desired hysteresis =
200mV, V
CC
= 5V.
Use R
DIV
= 150k
R
k
R
kV
3
150 2 5
124
302
5
301 5
02 001725
9
=
()
=
=
()
() ()
=
.
.
.–. .
k (use 301k, 1%)
.56M (Use 10M)
R
kM
Mk
4
301 10 1 24
10 2 5 1 24 301 5 1 24
=
()()()
−
()
+−
()
=
.
.. .
272k (Use 274k 1%)
The total divider resistance will be 274k + 301k = 575k, and
this will draw about 7µA from a fully charged battery.
Synchronizing
The SYNC pin on the LT1500 can be used to synchronize
switching frequency to an external clock. The pin should
be driven with a 50ns to 300ns pulse which will trigger the
switch to an on state. There is a fairly restricted range over
which synchronizing will work, because the period between
sync pulses must be greater than the natural on-time of
the regulator when it is running unsynchronized, and the
sync frequency must be greater than the unsynchronized
switching frequency. This puts the following restrictions
on synchronized operation:
ff
f
fV
VV
SYNC NATURAL
SYNC
NATURAL OUT
OUT IN
>
<
()
–
(Use Minimum V )
IN
f
NATURAL
is the natural unsynchronized switching fre-
quency of the regulator. It is a function of load current, so
a careful check must be done to ensure that the above
conditions are met under all load and input voltage condi-
tions.
Soft Start (SS)
The LT1500 can be soft started by connecting a capacitor
to the SS pin. This pin is the base of a PNP transistor
whose emitter is tied to the V
C
pin. Soft start action will
occur over the range of 0V to 0.8V on the SS pin and the
pin is clamped at 1.2V with an internal clamp. An internal
4µA pull-up current and the external capacitor value
determine soft start time. In a typical application a 0.22µF
capacitor is sufficient to limit input surges and prevent
output overshoot, even with overcompensation on the V
C
pin. Output voltages greater than 6V with very large output
