LTC3891 datasheet(13/40 Pages) LINER

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LTC3890-2

38902f

OPERATION (Refer to the Functional Diagram)

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping or Forced Continuous Mode)

(PLLIN/MODE Pin)

The LTC3890-2 can be enabled to enter high efficiency

Burst Mode operation, constant frequency pulse-skipping

mode, or forced continuous conduction mode at low load

currents. To select Burst Mode operation, tie the PLLIN/

MODE pin to a DC voltage below 0.8V (e.g., SGND). To

select forced continuous operation, tie the PLLIN/MODE

pin to INTVCC. To select pulse-skipping mode, tie the

PLLIN/MODE pin to a DC voltage greater than 1.2V and

less than INTVCC – 1.3V.

When a controller is enabled for Burst Mode operation, the

minimum peak current in the inductor is set to approxi-

mately 25% of the maximum sense voltage even though

the voltage on the ITH pin indicates a lower value. If the

average inductor current is higher than the load current,

the error amplifier, EA, will decrease the voltage on the

ITH pin. When the ITH voltage drops below 0.425V, the

internal sleep signal goes high (enabling sleep mode)

and both external MOSFETs are turned off. The ITH pin is

then disconnected from the output of the EA and parked

at 0.450V.

In sleep mode, much of the internal circuitry is turned off,

reducing the quiescent current that the LTC3890-2 draws.

If one channel is shut down and the other channel is in

sleep mode, the LTC3890-2 draws only 50μA of quiescent

current. If both channels are in sleep mode, the LTC3890-

2 draws only 60μA of quiescent current. In sleep mode,

the load current is supplied by the output capacitor. As

the output voltage decreases, the EA’s output begins to

rise. When the output voltage drops enough, the ITH pin

is reconnected to the output of the EA, the sleep signal

goes low, and the controller resumes normal operation

by turning on the top external MOSFET on the next cycle

of the internal oscillator.

When a controller is enabled for Burst Mode operation, the

inductor current is not allowed to reverse. The reverse cur-

rent comparator, IR, turns off the bottom external MOSFET

just before the inductor current reaches zero, preventing

it from reversing and going negative. Thus, the controller

operates in discontinuous operation.

In forced continuous operation or clocked by an external

clock source to use the phase-locked loop (see Frequency

Selection and Phase-Locked Loop section), the induc-

tor current is allowed to reverse at light loads or under

large transient conditions. The peak inductor current is

determined by the voltage on the ITH pin, just as in normal

operation. In this mode, the efficiency at light loads is lower

than in Burst Mode operation. However, continuous opera-

tion has the advantage of lower output voltage ripple and

less interference to audio circuitry. In forced continuous

mode, the output ripple is independent of load current.

When the PLLIN/MODE pin is connected for pulse-skipping

mode, the LTC3890-2 operates in PWM pulse-skipping

mode at light loads. 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 external 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). This mode, like forced continuous operation,

exhibits low output ripple as well as low audio noise and

reduced RF interference as compared to Burst Mode

operation. It provides higher low current efficiency than

forced continuous mode, but not nearly as high as Burst

Mode operation.

Frequency Selection and Phase-Locked Loop

(FREQ and PLLIN/MODE 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 frequency of the LTC3890-2’s controllers

can be selected using the FREQ pin.

If the PLLIN/MODE pin is not being driven by an external

clock source, the FREQ pin can be tied to SGND, tied to

INTVCC or programmed through an external resistor. Tying

FREQ to SGND selects 350kHz while tying FREQ to INTVCC

selects 535kHz. Placing a resistor between FREQ and SGND

allows the frequency to be programmed between 50kHz

and 900kHz, as shown in Figure 10.

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