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LTC3630

3630fb

OPERATION

The LTC3630 is a synchronous step-down DC/DC con-

verter with internal power switches that uses Burst Mode

control. The low quiescent current and high switching

frequency results in high efficiency across a wide range

of load currents. Burst Mode operation functions by using

short “burst” cycles to switch the inductor current through

the internal power MOSFETs, followed by a sleep cycle

where the power switches are off and the load current is

supplied by the output capacitor. During the sleep cycle,

the LTC3630 draws only 12μA of supply current. At light

loads, the burst cycles are a small percentage of the total

cycle time which minimizes the average supply current,

greatly improving efficiency. Figure 1 shows an example

of Burst Mode operation. The switching frequency and the

number of switching cycles during Burst Mode operation

are dependent on the inductor value, peak current, load

current, input voltage and output voltage.

reference, the comparator activates a sleep mode in which

the power switches and current comparators are disabled,

reducing the VIN pin supply current to only 12μA. As the

load current discharges the output capacitor, the voltage

on the VFB pin decreases. When this voltage falls 5mV

below the 800mV reference, the feedback comparator

trips and enables burst cycles.

At the beginning of the burst cycle, the internal high side

power switch (P-channel MOSFET) is turned on and the

inductor current begins to ramp up. The inductor current

increases until either the current exceeds the peak cur-

rent comparator threshold or the voltage on the VFB pin

exceeds 800mV, at which time the high side power switch

is turned off and the low side power switch (N-channel

MOSFET) turns on. The inductor current ramps down until

the reverse current comparator trips, signaling that the

current is close to zero. If the voltage on the VFB pin is

still less than the 800mV reference, the high side power

switch is turned on again and another cycle commences.

The average current during a burst cycle will normally be

greater than the average load current. For this architecture,

the maximum average output current is equal to half of

the peak current.

The hysteretic nature of this control architecture results

in a switching frequency that is a function of the input

voltage, output voltage, and inductor value. This behavior

provides inherent short-circuit protection. If the output is

shorted to ground, the inductor current will decay very

slowly during a single switching cycle. Since the high side

switch turns on only when the inductor current is near

zero, the LTC3630 inherently switches at a lower frequency

during start-up or short-circuit conditions.

Start-Up and Shutdown

If the voltage on the RUN pin is less than 0.7V, the LTC3630

enters a shutdown mode in which all internal circuitry is

disabled, reducing the DC supply current to 5μA. When the

voltage on the RUN pin exceeds 1.21V, normal operation

of the main control loop is enabled. The RUN pin com-

parator has 110mV of internal hysteresis, and therefore

must fall below 1.1V to stop switching and disable the

main control loop.

BURST

FREQUENCY

INDUCTOR

CURRENT

OUTPUT

VOLTAGE

ΔVOUT

3630 F01

BURST

CYCLE

SLEEP

CYCLE

SWITCHING

FREQUENCY

Figure 1. Burst Mode Operation

Main Control Loop

The LTC3630 uses the VPRG1 and VPRG2 control pins to

connect internal feedback resistors to the VFB pin. This

enables fixed outputs of 1.8V, 3.3V or 5V without increas-

ing component count, input supply current or exposure to

noise on the sensitive input to the feedback comparator.

External feedback resistors (adjustable mode) can still

be used by connecting both VPRG1 and VPRG2 to ground.

In adjustable mode the feedback comparator monitors

the voltage on the VFB pin and compares it to an inter-

nal 800mV reference. If this voltage is greater than the

(Refer to Block Diagram)

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