LTC1682-3.3 datasheet(8/12 Pages) LINER

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LTC1682/LTC1682-3.3/LTC1682-5

Operation

The LTC1682 uses a switched-capacitor charge pump to

generate a CPO voltage of approximately 2(VIN). CPO

powers an internal low dropout linear regulator that sup-

plies a regulated output at VOUT. Internal comparators are

used to sense CPO and VIN 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(VIN) the LDO is

enabled. If CPO falls below 1.45(VIN) 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(VIN) and IOUT 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(VIN)

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 VIN, 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. VOUT is set by an

external or internal resistor divider. The LDO requires a

capacitor on VOUT 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:

VOUT = 1.235V(1 + R1/R2)

The output voltage range is 2.5V to 5.5V.

Maximum VOUT and IOUT 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(VIN) (see Figure 4).

Figure 3. Powering an Auxiliary Regulator from CPO

APPLICATIONS INFORMATION

56k

18k

0.22

µF

4.7

µF

1682 F03

VIN

3.6V

4.7

µF

5.1V

VRIPPLE = 800µVP-P

VOUT

SHDN

GND

CPO

C+

VIN

C–

LTC1682

4.7

µF

3.3V

OUT

GND

EXTERNAL LDO

VOUT = 1.235V(1 + R1/R2)

–

2VIN

RCPO

CCPO

VCPO

VOUT

ILOAD

1682 F04

RDROPOUT

VDROPOUT

–

Figure 4. Equivalent Circuit

LTC1682-3.3 Datasheet(PDF) 8 Page - Linear Technology

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