MIC2132YML datasheet(35/48 Pages) MICROCHIP

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DS20006654B-page 35

MIC2132

On the other hand, the power dissipation in the

low-side power MOSFET in each phase channel is

mainly contributed to by the conduction loss. There is

no switching loss for the low-side MOSFET in the buck

converter, since the body diode of the low-side

MOSFET is forward-biased before the turn-on, and

after the turn-off of the low-side MOSFET, which makes

the voltage across the low-side MOSFET equal to the

body diode’s forward voltage during the turn-on and

turn-off transition.

Apart from the conduction loss, low-side MOSFET

body diode forward conduction loss, body diode

reverse recovery loss and low-side MOSFET output

capacitance discharge loss also contribute to the

power dissipation in the low-side power MOSFET in

each phase channel.

The low-side MOSFET body diode forward conduction

loss during dead time is calculated by Equation 5-32.

EQUATION 5-32:

The low-side MOSFET body diode reverse recovery

loss is calculated by Equation 5-33.

EQUATION 5-33:

The low-side MOSFET output capacitance discharge loss

can be calculated in Equation 5-34.

EQUATION 5-34:

The total power dissipation of the low-side power

MOSFET in each phase channel is estimated in

Equation 5-35.

EQUATION 5-35:

Since low-side MOSFETs can be accidentally turned

on by the high dV/dt signal at switching node, low-side

MOSFETs with high CGS/CGD ratio and low internal

gate resistance should be chosen to minimize the

effect of dV/dt inducted turn-on.

5.5

Bootstrap Capacitor

The MIC2132 device’s high-side gate drive circuits are

designed to switch the N-Channel external MOSFETs.

The Functional Block Diagram shows two internal

bootstrap diodes and each one is between the PVDD

and BST pins of each phase channel. These circuits

supply energy to the high-side gate drive circuits with

one for each phase. A low-ESR ceramic capacitor

should be connected between the BST pin and the SW

pin of each phase channel (refer to the “Typical Appli-

cation Circuit”). The bootstrap capacitors between

the BST and the SW pins, CBST1 and CBST2, are

charged while the respective low-side MOSFET is

turned on. When the respective high-side MOSFET

driver is turned on, energy from CBSTx is used to turn

the MOSFET on. A minimum of 0.1 μF low-ESR

ceramic capacitor is recommended between the BSTx

and SWx pins. The required value of CBSTX can be

calculated using Equation 5-36.

EQUATION 5-36:

5.6

Setting Output Voltage

The MIC2132 requires two resistors to set the output

voltage, as shown in Figure 5-3.

FIGURE 5-3:

Voltage-Divider

Configuration.

Where:

VF(BD) = Forward Voltage of Low-Side MOSFET

Body Diode

tDT = Dead Time, which is about 20 ns

PBDDT(LS) =

2 × IOUT(MAX)

× VF(BD) × tDT × fSW

Where:

QRR(BDLS) = Reverse Recovery Charge of Low-Side

MOSFET Body Diode

PBDQRR(LS) = VIN(MAX) × QRR(BDLS) × fSW

Where:

COSS(LS) = Low-Side MOSFET Output Capacitance

PCOSS(LS) = 0.5 × COSS(LS)(VIN(MAX))2 × fSW

PD(LS) = PCOND(LS) + PBDDT(LS) + PBDQRR(LS) + PCOSS(LS)

Where:

QG(HS) = Gate Charge of High-Side MOSFET in

Each Phase

∆VCBSTx = Delta Voltage Drop Across CBST in Each

Phase, Generally 50 mV to 100 mV

CBSTx =

QG(HS)

VCBSTx

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