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LTC1878

OPERATIO

Main Control Loop

The LTC1878 uses a constant frequency, current mode

step-down architecture. Both the main (P-channel

MOSFET) and synchronous (N-channel MOSFET) switches

are internal. During normal operation, the internal top

power MOSFET is turned on each cycle when the oscillator

sets the RS latch, and turned off when the current com-

parator, ICOMP, resets the RS latch. The peak inductor

current at which ICOMP resets the RS latch is controlled by

the voltage on the ITH pin, which is the output of error

amplifier EA. The VFB pin, described in the Pin Functions

section, allows EA to receive an output feedback voltage

from an external resistive divider. When the load current

increases, it causes a slight decrease in the feedback

voltage relative to the 0.8V reference, which in turn,

causes the ITH voltage to increase until the average induc-

tor current matches the new load current. While the top

MOSFET is off, the bottom MOSFET is turned on until

either the inductor current starts to reverse as indicated by

the current reversal comparator IRCMP, or the beginning of

the next clock cycle.

Comparator OVDET guards against transient overshoots

>6.25% by turning the main switch off and keeping it off

until the fault is removed.

Burst Mode Operation

The LTC1878 is capable of Burst Mode operation in which

the internal power MOSFETs operate intermittently based

on load demand. To enable Burst Mode operation, simply

tie the SYNC/MODE pin to VIN or connect it to a logic high

(VSYNC/MODE> 1.5V). To disable Burst Mode operation and

enable PWM pulse skipping mode, connect the SYNC/

MODE pin to GND. In this mode, the efficiency is lower at

light loads, but becomes comparable to Burst Mode

operation when the output load exceeds 50mA. The ad-

vantage of pulse skipping mode is lower output ripple and

less interference to audio circuitry.

When the converter is in Burst Mode operation, the peak

current of the inductor is set to approximately 250mA,

even though the voltage at the ITH pin indicates a lower

value. The voltage at the ITH pin drops when the inductor’s

average current is greater than the load requirement. As

the ITH voltage drops below approximately 0.45V, the

BURST comparator trips, causing the internal sleep line to

go high and forces off both power MOSFETs. The ITH pin

is then disconnected from the output of the EA amplifier

and parked a diode voltage above ground.

In sleep mode, both power MOSFETs are held off and a

majority of the internal circuitry is partially turned off,

reducing the quiescent current to 10

µA. The load current

is now being supplied solely from the output capacitor.

When the output voltage drops, the ITH pin reconnects to

the output of the EA amplifier and the top MOSFET is again

turned on and this process repeats.

Short-Circuit Protection

When the output is shorted to ground, the frequency of the

oscillator is reduced to about 80kHz, 1/7 the nominal

frequency. This frequency foldback ensures that the

inductor current has ample time to decay, thereby pre-

venting runaway. The oscillator’s frequency will progres-

sively increase to 550kHz (or the synchronized frequency)

when VFB rises above 0.3V.

Frequency Synchronization

A phase-locked loop (PLL) is available on the LTC1878 to

allow the internal oscillator to be synchronized to an

external source connected to the SYNC/MODE pin. The

output of the phase detector at the PLL LPF pin operates

over a 0V to 2.4V range corresponding to 400kHz to

700kHz. When locked, the PLL aligns the turn-on of the top

MOSFET to the rising edge of the synchronizing signal.

When the LTC1878 is clocked by an external source, Burst

Mode operation is disabled; the LTC1878 then operates in

PWM pulse skipping mode. In this mode, when the output

load is very low, current comparator ICOMP may remain

tripped for several cycles and force the main switch to stay

off for the same number of cycles. Increasing the output

load slightly allows constant frequency PWM operation to

resume. This mode exhibits low output ripple as well as

low audio noise and reduced RF interference while provid-

ing reasonable low current efficiency.

Frequency synchronization is inhibited when the feedback

voltage VFB is below 0.6V. This prevents the external clock

from interfering with the frequency foldback for short-

circuit protection.

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