LTC1517-3.3 Просмотр технического описания (PDF) - Linear Technology

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LTC1517-3.3

Micropower, Regulated 3.3V Charge Pump in a 5-Pin SOT-23 Package

Linear Technology 

LTC1517-3.3

APPLICATIONS INFORMATION

Operation

The LTC1517-3.3 uses a switched-capacitor charge pump

to boost VIN to a 3.3V ±4% regulated output. The part

achieves regulation by sensing the output voltage through

an internal resistor divider and enabling the charge pump

when the divided output droops below the comparator’s

lower trip point (set by VREF). When the charge pump is

enabled, a 2-phase nonoverlapping clock controls the

internal charge pump switches. Flying capacitor C1 is

charged to VIN on phase one of the clock. On phase two of

the clock, C1 is stacked in series with VIN and connected

to VOUT through an internal switch. This sequence of

charging and discharging the flying capacitor occurs at a

free running frequency of 700kHz (typ) and continues until

the divided output voltage reaches the upper trip point of

the comparator. Once the output is back in regulation, the

charge pump is disabled. This method of bursting the

charge pump on and off enables the LTC1517-3.3 to

achieve high efficiency at extremely low output loads.

Capacitor Selection

For best performance, it is recommended that low ESR

capacitors be used for both CIN and COUT to reduce noise

and ripple. The CIN and COUT capacitors should be either

ceramic or tantalum and should be 3.3µF or greater.

Ceramic capacitors will provide the smallest size for a

given capacitance. If the input source impedance is very

low (< 0.5Ω), CIN may not be needed. Ceramic capacitors

are recommended for the flying capacitor C1 with values

of 0.1µF or 0.22µF. Smaller value flying capacitors may be

used in low IOUT applications.

Output Ripple

Normal LTC1517-3.3 operation produces voltage ripple

on the VOUT pin. Output voltage ripple is required for the

parts to regulate. Low frequency ripple exists due to the

hysteresis in the sense comparator and propagation de-

lays in the charge pump enable/disable circuits. High

frequency ripple is also present mainly from the ESR

(equivalent series resistance) in the output capacitor.

Typical output ripple with VIN = 2.5V under maximum load

is 75mV peak-to-peak with a low ESR 3.3µF output capaci-

tor (minimum recommended COUT). For applications

requiring VIN to exceed 3.3V or for applications requiring

less than 75mV of peak-to-peak ripple, a 6.8µF to 10µF

COUT capacitor is recommended. Slight further decreases

in output ripple can be achieved by using COUT capacitors

larger than 10µF.

Short-Circuit/Thermal Protection

During short-circuit conditions, the LTC1517-3.3 will draw

between 20mA and 150mA from VIN, causing a rise in

junction temperature. On-chip thermal shutdown circuitry

disables the charge pump once the junction temperature

exceeds approximately 160°C. The charge pump is

reenabled once the junction temperature drops to approxi-

mately 145°C. The LTC1517-3.3 will cycle in and out of

thermal shutdown indefinitely without latchup or damage

until the VOUT short is removed.

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