MCP16311SLASH2 datasheet(15/40 Pages) MICROCHIP

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DS20005255C-page 15

MCP16311/2

4.2

Functional Description

4.2.1

STEP-DOWN OR BUCK

CONVERTER

The MCP16311/2 is a synchronous step-down or buck

converter capable of stepping input voltages ranging

from 4.4V to 30V down to 2.0V to 24V for VIN >VOUT.

The integrated high-side switch is used to chop or

modulate the input voltage using a controlled duty

cycle. The integrated low-side switch is used to

freewheel current when the high-side switch is turned

off. High efficiency is achieved by using low-resistance

switches and low equivalent series resistance (ESR)

inductors and capacitors. When the high-side switch is

turned on, a DC voltage is applied to the inductor (VIN –

VOUT), resulting in a positive linear ramp of inductor

current. When the high-side switch turns off and the

low-side switch turns on, the applied inductor voltage is

equal to –VOUT, resulting in a negative linear ramp of

inductor current. In order to ensure there is no shoot-

through current, a dead time where both switches are

off is implemented between the high-side switch

turning off and the low-side switch turning on, and the

low-side switch turning off and the high-side switch

turning on.

For

steady-state,

continuous

inductor

current

operation, the positive inductor current ramp must

equal the negative current ramp in magnitude. While

operating in steady state, the switch duty cycle must be

equal to the relationship of VOUT/VIN for constant

output voltage regulation, under the condition that the

inductor current is continuous or never reaches zero.

For discontinuous inductor current operation, the

steady-state duty cycle will be less than VOUT/VIN to

maintain voltage regulation. When the inductor current

reaches zero, the low-side switch is turned off so that

current does not flow in the reverse direction, keeping

the efficiency high. The average of the chopped input

voltage or SW node voltage is equal to the output

voltage, while the average inductor current is equal to

the output current.

FIGURE 4-2:

Synchronous Step-Down

Converter.

VIN

IOUT

VOUT

Continuous Inductor Current Mode

S1 ON

S2 ON

VIN

IOUT

Discontinuous Inductor Current Mode

S1 ON S2

Both

OFF

VIN

COUT

VOUT

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