LTC3336 datasheet(13/22 Pages) AD

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LTC3336

Rev. 0

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OPERATION

BUCK DC/DC REGULATOR

The buck regulator uses hysteretic voltage control to

regulate the output through internal feedback from the

VOUT sense pin. The output voltage can be programmed

to one of sixteen different fixed values. In dropout, the

P-channel MOSFET is turned on 100%.

The buck converter charges an output capacitor through

an inductor to a value slightly higher than the regulation

point. It does this by ramping the inductor current up to

the peak current threshold IPEAK (see Table 1) through

an internal P-channel MOSFET switch and then ramping

it down to the valley current threshold IVALLEY through

an internal N-channel MOSFET switch. See waveform in

Figure 1. This efficiently delivers energy to the output

capacitor. The ramp rates are determined by VIN, VOUT,

and the inductor value.

Table 1. IPEAK Selection

IPK1

IPK0

IPEAK

10mA

30mA

100mA

300mA

Figure 1.

IPEAK

IVALLEY

INDUCTOR

CURRENT

3336 F01

After the buck brings the output voltage into regulation,

the converter enters a low quiescent current sleep state

that monitors the output voltage with a sleep comparator.

The inductor current is ramped down to zero current in a

controlled manner before entering SLEEP.

During SLEEP, load current is provided by the buck output

capacitor. When the output voltage falls below the regu-

lation point, the buck regulator wakes up and the cycle

repeats. This hysteretic method of providing a regulated

output reduces losses associated with MOSFET switching

and maintains an output in regulation at light loads.

When the sleep comparator signals that the output has

reached the sleep threshold, the buck converter may be

in the middle of a cycle with current still flowing through

the inductor. Normally both synchronous switches would

turn off and the current in the inductor would freewheel

to zero through the N-channel MOSFET body diode. The

LTC3336 keeps the N-channel MOSFET switched on dur-

ing this time to prevent the conduction loss that would

occur in the diode if the N-channel MOSFET were off. If

the P-channel MOSFET is on when the sleep compara-

tor trips, the P-channel immediately turns off and the

N-channel MOSFET turns on.

Though the quiescent current when the buck is switching

is much greater than the sleep quiescent current, it is still

a small percentage of the average inductor current which

results in high efficiency over most load conditions.

The buck operates only when the output voltage dis-

charges below the sleep falling threshold. Thus, the buck

operating quiescent current is averaged with the low sleep

quiescent current. This allows the converter to remain

very efficient at loads of 10μA and lower.

Sixteen selectable voltages are available by tying the out-

put select bits (OUT3, OUT2, OUT1, OUT0) to INTVCC or

VOUT (for logic high) or to GND (for logic low). Table 2

shows the sixteen codes and their corresponding

output voltages.

OPERATION WITH BUCK SUPPLIED FROM OUTPUT

For output voltage settings ≥ 2.4V the quiescent current

of the buck regulator is taken from the regulated output

VOUT. This mode of operation significantly reduces the VIN

quiescent current by about a factor of VIN/VOUT.

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