MICROPOWER, RE GULATED 5V
CHARGE PUMP
© 2000 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
SC1517-5
PRELIMINARY - April 3, 2000
8
APPLICATION NOTES
THEORY OF OPERATION
The SC1517-5 utilizes a switched capacitor charge
pump, to generate an output voltage regulated to 5V+/-
4%.
An internal oscillator generates two out of phase c lock
signals which turn on and off internal MOSFET
switches. During phase1 of clock, the flying charge
pump c apacitor 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
Vout
Vin
Gnd
Gnd
Cin
CoutC_charge
S1 S4
S3 S2
S1,S2
S3,S4
ON
ON
OFF
OFF
Charge transfer from Cin to C_charge
Charge transfer from (Cin,and C_charge) to Cout
Current Limit
&
Soft Start
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
com parator sens es the output voltage and com pares it
to a signal approximating VIN - 500mV.
If VOUT < VIN - 500mV, the com parator enables a 65m A
typical limited current source and the charge pump is
disabled. Once the over current is removed and
VOUT > VIN - 500m V, the cur rent 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
temper ature exceeds appr oxim ately 160°C, the char ge
pump is disabled. The charge pum p 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.
C1+
C1-