LT3757 datasheet(17/48 Pages) AD

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LTC7841

Rev. 0

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OPERATION

Start-Up (SS Pin)

The start-up of the controller’s output voltage VOUT is

controlled by the voltage on the SS pin. When the volt-

age on the SS pin is less than the internal reference, the

LTC7841 regulates the VFB voltage to the SS pin voltage

instead of the reference. This allows the SS pin to be used

to program a soft-start by connecting an external capaci-

tor from the SS pin to SGND. An internal 10μA pull-up

current charges this capacitor creating a voltage ramp on

the SS pin. As the SS voltage rises linearly from 0V to the

reference (and beyond up to INTVCC), the output voltage

rises smoothly to its final value.

Light Load Current Operation—Burst Mode Operation,

Pulse-Skipping or Continuous Conduction

(PLLIN/MODE Pin)

The LTC7841 can be enabled to enter high efficiency

Burst Mode operation, constant-frequency, pulse-skip-

ping mode or forced continuous conduction mode at low

load currents. To select Burst Mode operation, tie the

PLLIN/MODE pin to ground (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 the controller is enabled for Burst Mode opera-

tion, the minimum peak current in the inductor is set to

approximately 30% 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 required

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.

In sleep mode much of the internal circuitry is turned off

and the LTC7841 draws only 800μA of quiescent current.

In sleep mode the load current is supplied by the output

capacitor. As the output voltage decreases, the EA’s out-

put begins to rise. When the output voltage drops enough,

the sleep signal goes low and the controller resumes nor-

mal operation by turning on the bottom external MOSFET

on the next cycle of the internal oscillator.

When the controller is enabled for Burst Mode opera-

tion, the inductor current is not allowed to reverse. The

reverse current comparator (IREV) turns off the top exter-

nal MOSFET just before the inductor current reaches zero,

preventing it from reversing and going negative. Thus, the

controller operates in discontinuous current operation.

In forced continuous operation or when clocked by an

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

the Frequency Selection and Phase-Locked Loop section),

the inductor current is allowed to reverse at light loads or

under large transient conditions. The peak inductor cur-

rent 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 operation has the advantages of lower output

voltage ripple and less interference to audio circuitry, as

it maintains constant-frequency operation independent

of load current.

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

ping mode, the LTC7841 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 bottom 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.

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