MOS6020 datasheet(17/34 Pages) MICROCHIP

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DS22285A-page 17

MCP16321/2

4.2

Functional Description

4.2.1

STEP-DOWN OR BUCK

CONVERTER

The MCP16321/2 is a synchronous, step-down or buck

converter capable of stepping input voltages ranging

from 6V to 24V down to 0.9V to 5V.

The integrated high-side switch is used to chop or

modulate the input voltage using a controlled duty cycle

for output voltage regulation. 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), inductor 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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