LTC7825

12

Rev. A

For more information www.analog.com

EXTVCC allows the MOSFET driver and control power to be

derived from other high efficiency sources such as VLOW

pin of a 20V to 10V voltage divider or other voltage rails

in the system. Using EXTVCC can significantly reduce the

IC temperature rise and improve the system efficiency in

high VIN applications. Do not apply more than 13V to the

EXTVCC pin.

FLOAT MOSFET DRIVER SUPPLY (CB)

External bootstrap capacitors CB1/CB2/CB3, connected to

the BOOST pins, supply the gate drive voltages for the

internal power MOSFETs. Capacitors in the Functional

Diagram are charged though internal diodes from

INTVCC. Typically, 0.1µF X7R is adequate for the boot-

strap capacitor.

UNDERVOLTAGE LOCKOUT

The LTC7825 has a precision UVLO comparator con-

stantly monitoring the INTVCC voltage to ensure that an

adequate gate-drive voltage is present. It locks out the

switching action when INTVCC is below 3.9V. To prevent

oscillation when there is a disturbance on the INTVCC, the

UVLO comparator has 300mV hysteresis.

INPUT DISCONNECT MOSFET AND OVG CONTROL

The LTC7825 features the optional input disconnect pro-

tection. This feature uses an external N-channel MOSFET,

the gate of which is connected to the OVG pin. The OVG

pin turns the MOSFET on and off. In normal operation, the

OVG pin voltage is 4V higher than the VHIGH pin regulated

by an internal charge pump. If the VHIGH pin voltage is

higher than 26V, the OVG pin is pulled down to VHIGH to

turn off the external MOSFET, and switching stops. The

LTC7825 can survive at an input transient voltage as high

as 40V which is limited by the VCC pin maximum voltage

rating. Refer to the Typical Performance Characteristics

section for the example of input overvoltage protection.

If the input voltage is always below 24V and the input

disconnect feature is not needed, let the OVG pin float.

OUTPUT DISCONNECT MOSFET AND SHORT-CIRCUIT

PROTECTIONS

The output disconnect MOSFET is integrated internally

and designed to protect the output voltage during the

input short-circuit conditions. The output disconnect

MOSFET behaves like an ideal diode from the VLOW to

the VOUT pin and it is turned off when the VHIGH voltage

is lower than the VOUT voltage. During VHIGH short-circuit

conditions, the VLOW voltage will be pulled down due to

the internal body diodes from VLOW to VHIGH; the VOUT

voltage will not be pulled down. If the output disconnect

function is not needed, users may short the VLOW and

VOUT pins together on the PCB to achieve better efficiency

and thermal performances.

FAULT RESPONSE AND TIMER PIN

The LTC7825 stops switching and pulls the FAULT pin low

during fault conditions. A capacitor connected from the

TIMER pin to GND sets the time for retry and start-up if

fault conditions are removed. A typical waveform on the

TIMER pin, during a fault condition, is shown in Figure 2.

After the FAULT pin is pulled low, a 4µA pull-up current

flows out of TIMER pin and starts to charge the TIMER

capacitor. The pull-up current increases to 8µA when the

TIMER pin voltage is higher than 0.5V and back to 4µA

when the TIMER pin voltage is higher than 1.0V. The

TIMER pin will be strongly pulled down whenever the

fault conditions are removed or the TIMER pin voltage is

higher than 3V. When the TIMER pin voltage is between

0.5V and 1.0V, the internal pre-balance circuit will source

or sink current to the VLOW pin and regulate the VLOW

pin to VHIGH/2 with around 200mA/50mA capability. The

pre-balance time can be calculated based on the value of

capacitor CTIMER on the TIMER pin.

For voltage divider applications, if the flying capacitor CFLY

and the VLOW capacitor are very large and input voltage

is high, it may take several pre-balance time periods to

pre-balance the VLOW pin to VHIGH/2 with a fixed CTIMER.

A longer start-up time is expected.

APPLICATIONS INFORMATION