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SC1517-5

MICROPOWER, REGULATED 5V CHARGE PUMP

Semtech Corporation 

MICROPOWER, REGULATED 5V

CHARGE PUMP

SC1517-5

October 3, 2000

APPLICATION NOTES

THEORY OF OPERATION

The SC1517-5 utilizes a switched capacitor charge

pump, to generate an output voltage regulated to 5V+/-

4%.

Current Limit

&

Soft Start

An internal oscillator generates two out of phase clock

signals which turn on and off internal MOSFET

switches. During phase1 of clock, the flying charge

Charge transfer from Cin to C_charge

ON

S1,S2

OFF

ON

S3,S4

OFF

Charge transfer from (Cin,and C_charge) to Cout

pump capacitor is charged to Vin through switches S1,

and S2. During phase2 the flying charge pump

capacitor is connected in series with the input voltage

via S3, and S4, producing an output voltage.

An integrated comparator senses the output voltage

Vin

Gnd

S1 C1+

C1-

S3

Cin

S4

C_charge

S2

Cout

Vout

Gnd

and disable the charge pump once an upper threshold

is reached. As the output voltage drops below the lower

threshold, the comparator enables the charge pump

and hence regulation is achieved.

SHORT CIRCUIT / SOFT START

An internal current limit and soft start circuit provides

protection against output short circuit and limits large

current spikes generated during the initial start up to

charge output capacitor.

As the over current condition is applied at the output, a

comparator senses the output voltage and compares it

to a signal approximating VIN - 500mV.

If VOUT < VIN - 500mV, the comparator enables a 65mA

typical limited current source and the charge pump is

disabled. Once the over current is removed and

VOUT > VIN - 500mV, the current source is disconnected

from output and the charge pump circuitry is enabled

again.

THERMAL SHUTDOWN

The thermal shutdown circuit provides added protection

to the SC1517-5. During a VOUT short circuit condition or

if the load is excessive (IOUT > = 20mA), the thermal

shut down circuit is active and if the junction

temperature exceeds approximately 160°C, the charge

pump is disabled. The charge pump is enabled only if

the junction temperature drops to approximately 145°C.

The SC1517-5 will cycle in and out of thermal shut

down until the short circuit or the excessive load

condition on VOUT is removed, thereby improving device

reliability.

VOUT RIPPLE & CAPACITOR SELECTION

The peak to peak output ripple voltage is dependent on

the following factors:

1- Oscillator Frequency

2- Hysteresis window of the comparator

3- ESR of CCharge , and Cout

4- Capacitance value of Cout , and CCharge

Item 1, and 2 are fixed and depend on the charge

pump device used. Item 3, and 4 can be optimized for

best performance and reduction of the output ripple.

© 2000 SEMTECH CORP.

8

652 MITCHELL ROAD NEWBURY PARK CA 91320

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