LTC1517CS5-5 Ver la hoja de datos (PDF) - Linear Technology

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LTC1517CS5-5

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

Linear Technology 

LTC1517-5

APPLICATIONS INFORMATION

Operation

The LTC1517-5 uses a switched-capacitor charge pump

to boost VIN to a 5V ±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 800kHz (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 LTC1517-5 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 and

lowest ESR 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 ca-

pacitor C1 with values of 0.1µF or 0.22µF. Smaller value

flying capacitors may be used in low IOUT applications.

Output Ripple

Normal LTC1517-5 operation produces voltage ripple on

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

to regulate. Low frequency ripple exists due to the hyster-

esis in the sense comparator and propagation delays 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 = 3V under maximum load is 100mV peak-to-peak

with a low ESR (< 0.5Ω) 3.3µF output capacitor (minimum

recommended COUT). For applications requiring VIN to

exceed 3.3V or for applications requiring < 100mV 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 LTC1517-5 will draw

between 50mA and 200mA 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 LTC1517-5 will cycle in and out of

thermal shutdown indefinitely without latchup or damage

until the VOUT short is removed.

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