Datasheet

8
LTC1682/LTC1682-3.3/LTC1682-5
Operation
The LTC1682 uses a switched-capacitor charge pump to
generate a CPO voltage of approximately 2(V
IN
). CPO
powers an internal low dropout linear regulator that sup-
plies a regulated output at V
OUT
. Internal comparators are
used to sense CPO and V
IN
voltages for power-up condi-
tioning. The output current is sensed to determine the
charge pump operating mode. A trimmed internal bandgap
is used as the voltage reference and a trimmed internal
oscillator is used to control the charge pump switches.
The charge pump is a doubler configuration that uses one
external flying capacitor. When enabled, a 2-phase
nonoverlapping clock controls the charge pump switches.
At start-up, the LDO is disabled and the load is removed
from CPO. When CPO reaches 1.75(V
IN
) the LDO is
enabled. If CPO falls below 1.45(V
IN
) the LDO will be
disabled. Generally, the charge pump runs open loop with
continuous clocking for low noise. If CPO is greater than
1.95(V
IN
) and I
OUT
is less than 100µA, the charge pump
will operate in Burst Mode operation for increased effi-
ciency but slightly higher output noise. In Burst Mode
operation, the clock is disabled when CPO reaches 1.95(V
IN
)
and enabled when CPO droops by about 100mV. The
switching frequency is precisely controlled to ensure that
the frequency is above 455kHz and at the optimum rate to
ensure maximum efficiency. The switch edge rates are
also controlled to minimize noise. The effective output
resistance at CPO is dependent on the voltage at V
IN
, CPO
and the junction temperature. A low ESR capacitor of ≥2µF
should be used at CPO for minimum noise.
The LDO is used to filter the ripple on CPO and to set an
output voltage independent of CPO. V
OUT
is set by an
external or internal resistor divider. The LDO requires a
capacitor on V
OUT
for stability and improved load transient
response. A low ESR capacitor of ≥2µF should be used.
Output Voltage Selection
The LTC1682-3.3/LTC1682-5 versions have internal
resistor networks to set the regulated output voltage. The
LTC1682 output voltage is set using an external resistor
divider (see Figure 3). The output voltage is determined
using the following formula:
V
OUT
= 1.235V(1 + R1/R2)
The output voltage range is 2.5V to 5.5V.
Maximum V
OUT
and I
OUT
Calculations
The maximum available output voltage and current can be
calculated based on the open circuit CPO voltage, the
dropout voltage of the LDO and the effective output
resistance of the charge pump. The open circuit CPO
voltage is approximately 2(V
IN
) (see Figure 4).
Figure 3. Powering an Auxiliary Regulator from CPO
APPLICATIONS INFORMATION
WUU
U
R1
56k
R2
18k
C1
0.22µF
C4
4.7µF
1682 F03
V
IN
3.6V
C3
4.7µF
5.1V
V
RIPPLE
= 800µV
P-P
1
2
3
4
8
7
6
5
V
OUT
SHDN
FB
GND
CPO
C
+
V
IN
C
–
LTC1682
C2
4.7µF
IN
3.3V
OUT
GND
C5
EXTERNAL LDO
V
OUT
= 1.235V(1 + R1/R2)
+
–
2V
IN
R
CPO
C
CPO
V
CPO
V
OUT
I
LOAD
1682 F04
R
DROPOUT
V
DROPOUT
+
–
Figure 4. Equivalent Circuit