LTC4366 datasheet(14/32 Pages) AD

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LTC7862

Rev 0

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OPERATION

beyond), the output voltage VOUT rises smoothly from

zero to its final value.

Light Load Current Operation (Pulse-Skipping Mode)

In switching mode, LTC7862 enters constant frequency

pulse-skipping mode at light load currents.

In this mode, constant frequency operation is maintained

down to approximately 1% of designed maximum output

current. At very light loads, the current comparator, ICMP,

may remain tripped for several cycles and force the exter-

nal top MOSFET to stay off for the same number of cycles

(i.e., skipping pulses). The inductor current is not allowed

to reverse (discontinuous operation).

Frequency Selection (FREQ Pin)

The selection of switching frequency is a trade-off between

efficiency and component size. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to maintain low output ripple voltage.

The switching frequency of the LTC7862 can be selected

using the FREQ pin.

The FREQ pin can be tied to GND, tied to INTVCC or pro-

grammed through an external resistor. Tying FREQ to

GND selects 350kHz while tying FREQ to INTVCC selects

535kHz. Placing a resistor between FREQ and GND allows

the frequency to be programmed between 50kHz and

900kHz, as shown in Figure 1.

Table 1 summarizes the different states in which the FREQ

pin can be used.

Table 1.

FREQ PIN

FREQUENCY

350kHz

INTVCC

535kHz

Resistor to GND

50kHz to 900kHz

Input Supply Overvoltage Lockout (OVLO Pin)

The LTC7862 implements a protection feature that stops

all switching and turns off both the top and bottom exter-

nal MOSFETs when the input voltage rises above a pro-

grammable operating range. By using a resistor divider

from the input supply to ground, the OVLO pin serves as

a precise input supply voltage monitor. Switching is dis-

abled when the OVLO pin rises above 1.2V, which can be

configured to limit switching to a specific range of input

supply voltage.

When switching is disabled, the LTC7862 can safely sus-

tain input voltages up to the absolute maximum rating of

150V. Input supply overvoltage events trigger a soft-start

reset, which results in a graceful recovery from an input

supply transient.

Output Overvoltage Protection

An overvoltage comparator guards against transient over-

shoots as well as other more serious conditions that may

overvoltage the output. When the VFB pin rises by more

than 10% above its regulation point of 0.800V, the top

MOSFET is turned off and the bottom MOSFET is turned

on until the overvoltage condition is cleared.

FREQ PIN RESISTOR (k)

600

800

1000

35 45 55

7862 F01

400

200

500

700

900

300

100

65 75 85 95 105 115 125

Figure 1. Relationship Between Oscillator Frequency

and Resistor Value at the FREQ Pin

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