5
LTC1262
APPLICATIONS INFORMATION
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Operation
The LTC1262 uses a charge pump tripler to generate 12V
from a V
CC
of 5V. The charge pump operates when clocked
by a 300kHz oscillator. When the oscillator output is low,
C1 and C2 are connected between V
CC
and GND, charging
them to V
CC
. When the oscillator output goes high, C1 and
C2 are stacked in series with the bottom plate of C1 pulled
to V
CC
. The top plate of C2 is switched to charge C
OUT
and
V
OUT
rises. V
OUT
is regulated to within 5% of 12V by an
oscillator pulse gating scheme. A resistor divider senses
V
OUT
. When the output of the divider (V
DIV
) is less than the
output of a bandgap (V
BGAP
) by the hysteresis voltage
(V
HYST
) of the comparator, oscillator pulses are applied to
the charge pump to raise V
OUT
. When V
DIV
is above V
BGAP
by V
HYST
, the oscillator pulses are prevented from clock-
ing the charge pump. V
OUT
drops until V
DIV
is below V
BGAP
by V
HYST
again. The gates of all internal switches are
driven between V
OUT
and GND. An internal diode ensures
that the LTC1262 will start up under load by charging C
OUT
to one diode drop below V
CC
.
To reduce supply current the LTC1262 may be put into
shutdown mode by floating the SHDN pin or taking it to
V
CC
. In this mode the bandgap, comparator, oscillator and
resistor divider are switched off to reduce supply current
to typically 0.5µA. At the same time an internal switch
shorts V
OUT
to V
CC
; V
OUT
takes 3.3ms to reach 5.1V (see
t
OFF
in Figure 1). When the SHDN pin is low, the LTC1262
exits shutdown and the charge pump operates to raise
V
OUT
to 12V. V
OUT
takes 500µs to reach the lower regula-
tion limit of 11.4V (see t
ON
in Figure 1).
Choice of Capacitors
The LTC1262 is tested with the capacitors shown in
Figure 2. C1 and C2 are 0.22µF ceramic capacitors and
C
IN
and C
OUT
are 4.7µF tantalum capacitors. Refer to
Table 1 if other choices are desired.
Table 1. Recommended Capacitor Types and Values
CAPACITOR CERAMIC TANTALUM ALUMINUM
C1, C2 0.22µF to 1µF Not Not
Recommended Recommended
C
OUT
2µF (Min) 4.7µF (Min) 10µF (Min)
C
IN
1µF (Min) 4.7µF (Min) 10µF (Min)
C1 and C2 should be ceramic capacitors with values in the
range of 0.22µF to 1µF. Higher values provide better load
regulation. Tantalum capacitors are not recommended as
the higher ESR of these capacitors degrades performance
when the load current is above 25mA with V
CC
= 4.75V.
C
IN
and C
OUT
can be ceramic, tantalum or electrolytic
capacitors. The ESR of C
OUT
introduces steps in the V
OUT
waveform whenever the charge pump charges C
OUT
. This
tends to increase V
OUT
ripple. Ceramic or tantalum capaci-
tors are recommended for C
OUT
if minimum ripple is
desired. The LTC1262 does not require a 0.1µF capacitor
between V
CC
and V
OUT
for stability.
Maximum Load Current
The LTC1262 will source up to 50mA continuously without
any damage to itself. Do not short the V
OUT
pin to ground.
If the V
OUT
pin is shorted to ground, irreversible damage
to the device will result.
