7

LTC1751/LTC1751-3.3/LTC1751-5

APPLICATIO S I FOR ATIO

Operation (Refer to Simplified Block Diagrams)

The LTC1751 family uses a switched capacitor charge

tion is achieved by sensing the output voltage through a

resistor divider and enabling the charge pump when the

divided output drops below the lower trip point of COMP1.

When the charge pump is enabled, a 2-phase

nonoverlapping clock activates the charge pump switches.

clock. On phase 2 of the clock, it is stacked in series with

discharging the flying capacitor continues at the clock

frequency until the divided output voltage reaches the

upper trip point of COMP1. Once this happens the charge

pump is disabled. When the charge pump is disabled the

device typically draws less than 20

providing high efficiency under low load conditions.

In shutdown mode all circuitry is turned off and the

pin is a CMOS input with a threshold voltage of approxi-

mately 0.8V. The LTC1751 is in shutdown when a logic low

is applied to the SHDN pin. The quiescent supply current

of the LTC1751 will be slightly higher if the SHDN pin is

impedance CMOS input it should never be allowed to float.

To ensure that its state is defined it must always be driven

with a valid logic level.

Power Efficiency

The efficiency (

η) of the LTC1751 family is similar to that

of a linear regulator with an effective input voltage of twice

the actual input voltage. This occurs because the input

current for a voltage doubling charge pump is approxi-

mately twice the output current. In an ideal regulated

doubler the power efficiency would be given by:

η=

=

=

P

P

VI

VI

V

V

OUT

IN

OUT

OUT

IN

OUT

OUT

IN

•

•2

2

At moderate to high output power, the switching losses

and quiescent current of the LTC1751 are negligible and

the expression is valid. For example, an LTC1751-5 with

measured efficiency of 82% which is in close agreement

with the theoretical 83.3% calculation. The LTC1751 prod-

uct family continues to maintain good efficiency even at

fairly light loads because of its inherently low power

design.

Short-Circuit/Thermal Protection

During short-circuit conditions, the LTC1751 will draw

junction temperature. On-chip thermal shutdown circuitry

disables the charge pump once the junction temperature

exceeds approximately 160

°C and re-enables the charge

pump once the junction temperature drops back to ap-

proximately 150

°C. The device will cycle in and out of

thermal shutdown indefinitely without latchup or damage

The style and value of capacitors used with the LTC1751

family determine several important parameters such as

output ripple, charge pump strength and minimum

start-up time.

To reduce noise and ripple, it is recommended that low

ESR (< 0.1

These capacitors should be either ceramic or tantalum and

should be 6.8

µF or greater. Aluminum capacitors are not

recommended because of their high ESR. If the source

may not be needed. Alternatively, a somewhat smaller

value of input capacitor may be adequate, but will not be

the output ripple at the expense of higher minimum turn on

time and higher start-up current. See the section Output

Ripple.