LTC3201

6

3201f

Power Efficiency

The power efficiency (

η) of the LTC3201 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 regulator 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 LTC3201 are relatively low. Due

to the high clocking frequency, however, the current used

for charging and discharging the switches starts to reduce

power delivered will remain low.

Short-Circuit/Thermal Protection

The LTC3201 has short-circuit current limiting as well as

overtemperature protection. During short-circuit condi-

tions, the output current is limited to typically 150mA.

On-chip thermal shutdown circuitry disables the charge

pump once the junction temperature exceeds approxi-

mately 160

°C and re-enables the charge pump once the

junction temperature drops back to approximately 150

°C.

The LTC3201 will cycle in and out of thermal shutdown

indefinitely without latchup or damage until the short-

The style and value of capacitors used with the LTC3201

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 ceramic.

the output ripple at the expense of higher turn-on times

and start-up current. See the section Output Ripple. A 1

µF

should be a high frequency 0.22

µF capacitor with a reso-

a 1

µF ceramic capacitor with a resonant frequency over

1MHz. The X5R capacitor is a good choice for both. The

high and low frequency input current filtering. A higher

filter cap value will result in lower low frequency input

current ripple, but with increased high frequency ripple.

The key at the FILTER node is that the capacitor has to be

very high frequency. If capacitor technology improves the

bandwidth, then higher values should be used. Similarly,

increasing the input capacitor value but decreasing its

resonant frequency will not really help. Decreasing it will

help the high frequency performance while increasing the

low frequency current ripple.

Direct Connection to Battery

Due to the ultra low input current ripple, it is possible to

connect the LTC3201 directly to the battery without using

regulators or high frequency chokes.

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitor since its

voltage can reverse upon start-up. Low ESR ceramic

capacitors should always be used for the flying capacitor.

The flying capacitor controls the strength of the charge

pump. In order to achieve the rated output current it is

necessary to have at least 0.22

µF of capacitance for the

flying capacitor. Capacitors of different materials lose

their capacitance with higher temperature and voltage at

different rates. For example, a ceramic capacitor made of

X7R material will retain most of its capacitance from

–40

°C to 85°C whereas a Z5U and Y5V style capacitor will

lose considerable capacitance over that range. Z5U and

Y5V capacitors may also have a very strong voltage

coefficient causing them to lose 60% or more of their

capacitance when the rated voltage is applied. Therefore,

when comparing different capacitors it is often more

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