LTC3252

6

3252f

OPERATIO

The LTC3252 has two switched capacitor charge pumps to

charge pumps operate 180

° out of phase to reduce input

ripple. Regulation is achieved by sensing each output

voltage through an external resistor divider and modulat-

ing the charge pump output current based on the error

signal. A 2-phase nonoverlapping clock activates the two

charge pumps running them out of phase from each other.

On the first phase of the clock current is transferred from

switches of charge pump 1. Not only is current being

delivered to OUT1 on the first phase, but the flying capaci-

tor is also being charged up. On the second phase of the

clock, flying capacitor 1 is connected from OUT1 to

ground, transferring the charge stored during the first

phase of the clock to OUT1 via the switches of charge

pump 1. Charge pump 2 operates in the same manner to

supply current to OUT2, but with the phases of the clock

reversed relative to charge pump 1. Using this method of

switching, only half of the output current for each output

efficiency over a conventional LDO. A spread spectrum

oscillator, which utilizes random switching frequencies

between 1MHz and 1.6MHz, sets the rate of charging and

discharging of the flying capacitors. This architecture

achieves extremely low output noise. Input noise is signifi-

cantly reduced compared to conventional charge pumps.

The outputs also have a low current burst mode to improve

efficiency even at light loads.

In shutdown mode all circuitry is turned off and the

The EN1 and EN2 pins are CMOS inputs with threshold

voltages of approximately 0.8V to allow regulator control

with low voltage logic levels. The LTC3252 is in shutdown

when a logic low is applied to both enable pins. Since the

mode pins are high impedance CMOS inputs, they should

never be allowed to float. Always drive the enable pins with

valid logic levels.

(Refer to Simplified Block Diagram)

Short-Circuit/Thermal Protection

The LTC3252 has built-in short-circuit current limiting as

well as over temperature protection. During short-circuit

conditions, internal circuitry automatically limits each

output to approximately 500mA of current. If fault condi-

tions (such as shorted outputs) cause excessive self

heating on chip such that the junction temperature ex-

ceeds approximately 160

°C, the thermal shutdown cir-

cuitry will disable the charge pumps. The IC resumes

operation once the junction temperature drops back to

approximately 155

°C. The LTC3252 will cycle in and out of

thermal shutdown without latchup or damage until the

overstress condition is removed. Long term overstress

avoided as it can degrade the performance or shorten the

life of the part.

Soft-Start

the LTC3252 has built-in soft-start circuitry on each out-

put. When an output is enabled, the soft-start circuitry

increases the amount of current available from the output

linearly over a period of approximately 500

µs. The soft-

start circuitry is disabled shortly after the output achieves

regulation.

Spread Spectrum Operation

Switching regulators can be particularly troublesome where

electromagnetic interference (EMI) is concerned. Switch-

ing regulators operate on a cycle-by-cycle basis to transfer

power to an output. In most cases, the frequency of

operation is either fixed or is a constant based on the

output load. This method of conversion creates large

components of noise at the frequency of operation (funda-

mental) and multiples of the operating frequency (har-

monics).