LTC7825AVPBF datasheet(10/19 Pages) AD

10 / 19 page

LTC7825

Rev. A

For more information www.analog.com

OVERCURRENT PROTECTION

During the steady-state operation, the VLOW voltage drops

linearly as load current increases. The overcurrent pro-

tection can be set by choosing different VLOW to VHIGH/2

windows controlled by HYS_PRGM pin. If overcurrent

fault is triggered, the LTC7825 stops switching and will

retry to start up.

In practice, the VLOW voltage valley is impacted by switch-

ing frequency and CFLY/COUT. To set the correct hysteresis

window, measure the VLOW voltage at full load and maxi-

mum switching ripple. Although the LTC7825 is designed

to optimize efficiency around 10A load, there is no strict

limit on the output current. As long as the hysteresis win-

dow on the VLOW is large enough, the LTC7825 can sup-

ply more than 12A load current with heat sinks or forced

air flow. Refer to the Typical Performance Characteristics

section for an example of HYS_PRGM setting.

FREQUENCY SELECTION (FREQ PIN)

The selection of switching frequency is a trade-off between

efficiency and component size. Low frequency operation

increases efficiency by reducing MOSFET switching losses

but requires larger capacitance to maintain low output rip-

ple voltage and low output impedance. The FREQ pin can

be used to program the converter’s operating frequency

from 100kHz to 1.4MHz. There is a precision 10µA current

flowing out of the FREQ pin, so the user can program the

converter’s switching frequency with a single resistor to

SGND. The voltage on the FREQ pin is equal to the resis-

tance multiplied by 10μA current (e.g., the voltage is 1V

with a 100k resistor from the FREQ pin to GND). A curve

is provided in Figure 1 showing the relationship between

the voltage on the FREQ pin and switching frequency. Tie

FREQ pin to SGND/INTVCC enables the default switching

frequency of 300kHz/600kHz.

OPERATION

Figure 1. Relationship Between Switching

Frequency and Voltage at FREQ Pin

FREQ PIN VOLTAGE (V)

200

400

600

800

1000

1200

1400

1600

78251 F01

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