LTC7825

9

Rev. A

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OPERATION

MAIN CONTROL

The LTC7825 is a constant-frequency, open loop switched

capacitor/charge pump converter for high power and

high voltage applications. In the steady-state operation,

the internal switches are turned on and off to force the

flying capacitor between SWH and SWL either parallel

with the capacitor at VLOW or serial with the capacitor at

VLOW. When the flying capacitors and VLOW capacitors

are paralleled, the voltages on the flying capacitor and

VLOW capacitor are the same. When the flying capaci-

tors and VLOW capacitors are serial to VHIGH, the volt-

age on the flying capacitor plus the voltage on the VLOW

capacitor equals the voltage on VHIGH. So the VLOW pin

voltage is always close to half of the VHIGH voltage in the

steady state and it is not sensitive to the variable loads

due to the very low output impedance operating at a high

switching frequency. The LTC7825 does not regulate

the output voltage with a closed-loop feedback system.

However, it stops switching when fault conditions occur,

such as a VLOW pin short to GND, an overcurrent event,

and overtemperature conditions.

INTVCC/EXTVCC POWER

Power for the internal quad N-channel MOSFET driv-

ers and most other internal circuitry is derived from the

INTVCC pin. Normally an internal 4.5V linear regulator

supplies INTVCC power from VCC. If VCC is connected to

a high input voltage, an optional external voltage source

may be connected to the EXTVCC pin, enabling the second

4.5V linear regulator and supplying INTVCC power from

the EXTVCC pin. To enable the linear regulator under the

EXTVCC pin, VCC has to be higher than 6V and the EXTVCC

pin voltage has to be higher than 5V. Do not exceed 13V

on the EXTVCC pin. Each top MOSFET driver is biased

from the floating bootstrap capacitors CB, which normally

recharge during each off cycle through an internal diode

when the respective top MOSFET turns off.

START-UP AND SHUTDOWN (RUN)

The LTC7825 is in the shutdown mode when the RUN

pin is pulled down. In shutdown mode, most internal cir-

cuitry is turned off including the INTVCC regulator and the

LTC7825 consumes less than 30μA current. All internal

N-channel MOSFETs are actively turned off. Releasing

RUN allows an internal 1µA current to pull-up this pin

and enable the converter. Alternately, the RUN pin may

be externally pulled up or driven directly by logic. Do not

exceed the absolute maximum rating of 6V on this pin.

After RUN is released and the INTVCC voltage passes

UVLO, the LTC7825 starts up and monitors the VHIGH and

VLOW voltage continuously. The LTC7825 starts switching

and provides full power only if VLOW voltage is close to

half of VHIGH voltage or both VLOW and VHIGH voltages are

close to ground. In voltage divider applications, VLOW is

pre-balanced to half the VHIGH voltage and the LTC7825

may start-up with VLOW at different initial conditions.

FAULT PROTECTION AND THERMAL SHUTDOWN

The LTC7825 monitors system voltage, current and tem-

perature for fault protections. It will stop switching and

pull the FAULT pin down when fault conditions occur. To

clear VLOW voltage faults, the VLOW pin voltage has to

be within the programmed window around VHIGH/2 or

the VHIGH/VLOW voltages must be lower than 1V/0.5V. To

clear VHIGH over voltage fault, the VHIGH voltage must be

lower than 26V and OVG must be 4V higher than VHIGH.

To clear temperature faults, the IC temperature has to be

lower than 165°C.

The FAULT pin is allowed to be pulled up by an external

resistor to sources of up to 25V. It can be used to control

external disconnect MOSFETs, which isolate the input and

output during fault conditions.