5

LTC3801/LTC3801B

3801f

OPERATIO

(Refer to the Functional Diagram)

Main Control Loop (Normal Operation)

The LTC3801/LTC3801B are constant frequency current

mode step-down switching regulator controllers. During

normal operation, an external P-channel MOSFET is turned

on each cycle when the oscillator sets the RS latch and

turned off when the current comparator resets the latch.

The peak inductor current at which the current comparator

is the output of the error amplifier. The negative input to

which is generated by an external resistor divider con-

to increase until the average inductor current matches the

new load current.

internal 1

µA current source (2µA on LTC3801B) to charge

RUN pin voltage reaches approximately 0.6V, the main

up by a clamp to its zero current level of approximately

one diode voltage drop (0.7V). As the external compensa-

tion network continues to charge up, the corresponding

peak inductor current level follows, allowing normal op-

eration. The maximum peak inductor current attainable is

current level (approximately 1.9V).

Burst Mode Operation (LTC3801 Only)

The LTC3801 incorporates Burst Mode operation at low

clamp sets the peak current of the inductor at a level cor-

current level (approximately 1V), even though the actual

current is greater than the load requirement, the voltage at

to 0.15V above its zero current level (approximately 0.85V),

the sleep comparator will trip, turning off the external

MOSFET. In sleep, the input DC supply current to the IC is

reduced to 16

µAfrom195µAinnormaloperation.Withthe

switch held off, average inductor current will decay to zero

and the load will eventually cause the error amplifier out-

put to start drifting higher. When the error amplifier output

rises to 0.225V above its zero current level (approximately

0.925V), the sleep comparator will untrip and normal op-

eration will resume. The next oscillator cycle will turn the

external MOSFET on and the switching cycle will repeat.

Low Load Current Operation (LTC3801B Only)

voltage will be very close to the zero current level of 0.85V.

As the load current decreases further, an internal offset at

the current comparator input will ensure that the current

comparator remains tripped (even at zero load current)

and the regulator will start to skip cycles, as it must, in

order to maintain regulation. This behavior allows the

regulator to maintain constant frequency down to very

light loads, resulting in less low frequency noise genera-

tion over a wide load current range.

Figure 1 illustrates this result for the circuit on the front

page of this data sheet using both an LTC3801 (in Burst

Mode operation) and an LTC3801B (with Burst Mode

operation disabled). At an output current of 100mA, the

lower output current levels, the improvement is even

greater. This comes at a tradeoff of lower efficiency for the

non Burst Mode part at light load currents (see Figure 2).

Also notice the constant frequency operation of the

LTC3801B, even at 5% of maximum output current.

Dropout Operation

When the input supply voltage decreases towards the

output voltage, the rate of change of inductor current

during the on cycle decreases. This reduction means that

at some input-output differential, the external P-channel

MOSFET will remain on for more than one oscillator cycle

(start dropping off-cycles) since the inductor current has

not ramped up to the threshold set by the error amplifier.

Further reduction in input supply voltage will eventually

cause the external P-channel MOSFET to be turned on

100%, i.e., DC. The output voltage will then be determined

by the input voltage minus the voltage drop across the

sense resistor, the MOSFET and the inductor.