LTC7813 datasheet(13/28 Pages) AD

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LTC7878

Rev. 0

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OPERATION

cycles and then keep skipping pulses for longer time at

light loads until the control loop pull the ITH/ITHB high.

The pulse-skipping mode decreases the switching fre-

quency thus improves efficiency but may cause larger

output voltage ripple at light loads.

OUTPUT OVERVOLTAGE OPERATION

If the output voltage is higher than the value commanded

by the VFB resistor divider, the LTC7878 will respond

according to the mode and region of operation. In con-

tinuous conduction mode, the LTC7878 will sink current

into the input. If the input supply is capable of sinking cur-

rent, the LTC7878 will allow the reverse inductor current

to be sunk into the input. The maximum valley current of

the reverse inductor current is approximately 40% of the

maximum peak inductor current in the positive direction.

In pulse-skipping mode, switching will stop and the out-

put will be allowed to remain high.

CONSTANT-CURRENT REGULATION (CSP/CSN/

SETCUR PINS)

The LTC7878 provides a constant-current regulation

loop for either input or output average current. A sens-

ing resistor close to the input or output capacitor can

be used to sense the input or output current. Because

the input or output current may be a pulse current in the

different operation regions, A RC filter has to be applied

on CSP and CSN pins for average current sensing. When

the voltage on the current sensing resistor exceeds the

programmed current limit, the voltage on the ITH/ITHB

pin will be pulled low to decrease the inductor current and

maintain the desired maximum input or output current.

The current limit may be set by the voltage on the SETCUR

pin from 0.2V to 1.2V corresponding to the linearly 0mV

to 50mV current limit on the sensing resistor. There is a

15µA current out of the SETCUR pin and if the SETCUR

pin is float or SETCUR pin voltage higher than 1.2V, the

current limit is clamped at 50mV internally. The input

current limit function prevents overloading the DC input

source, while the output current limit provides a building

block for battery charger or LED driver applications. It

can also serve as an extra current limit protection for a

constant-voltage regulation application. The input/output

current limit function has an operating voltage range of

GND to the absolute maximum VOUT/VIN (72V).

FREQUENCY SELECTION AND PHASE-LOCKED LOOP

(FREQ, SYNC, PHASMD AND CLKOUT PINS)

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 fre-

quency of the LTC7878’s controllers can be selected using

the FREQ pin. If the SYNC pin is not being driven by an

external clock source, the FREQ pin can be used to pro-

gram the controller’s operating frequency from 100kHz to

600kHz. Switching frequency is determined by the voltage

on the FREQ pin. Since there is a precision 10μA current

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

controller’s switching frequency with a single resistor to

SGND (e.g., the FREQ pin voltage is 1V with 100k resis-

tor from the FREQ pin to SGND). A curve in Figure 2 is

provided to show the relationship between the voltage on

the FREQ pin and the switching frequency.

FREQ PIN VOLTAGE (V)

0.5

1.5

2.5

100

200

300

400

500

600

700

800

900

7878 F02

Figure 2. Relationship Between Switching

Frequency and Voltage at FREQ Pin

A phase-locked loop (PLL) is integrated on the LTC7878

to synchronize the internal oscillator to an external clock

source driving the SYNC pin. The PLL is capable of

locking to any frequency within the range of 100kHz to

600kHz. The frequency setting resistor at FREQ pin should

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