MIC2133 datasheet(37/50 Pages) MICROCHIP

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DS20006653B-page 37

MIC2133

low-side power MOSFET in each phase channel is

mainly contributed by the conduction loss, and there is

no switching loss for the low-side MOSFET in buck

converter because 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, and this makes

the voltage across the low-side MOSFET just equal to

the body diode forward voltage during the turn-on and

turn-off transition.

Apart from the conduction loss, the low-side MOSFET

body diode forward conduction loss, body diode

reverse recovery loss and low-side MOSFET output

capacitance discharge loss also contributed 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 the equation

below.

EQUATION 5-32:

The low-side MOSFET body diode reverse recovery

loss is calculated by the equation below.

EQUATION 5-33:

The low-side MOSFET output capacitance discharge loss

can be calculated in the equation below.

EQUATION 5-34:

The total power dissipation of the low-side power MOS-

FET in each phase channel is estimated in the equation

below.

EQUATION 5-35:

Low-side MOSFETs can be accidentally turned on by

the high dV/dt signal at the switching node; therefore, it

is recommended that low-side MOSFETs with a high

CGS/CGD ratio and low internal gate resistance be cho-

sen to minimize the effect of dV/dt inducted turn-on.

5.5

Bootstrap Capacitor

The MIC2133 device’s high-side gate drive circuits are

designed to switch the N-Channel external MOSFETs.

The MIC2133 “Functional Block Diagram” shows two

internal bootstrap diodes and each one is between the

PVDD and BST pins of each phase channel. These cir-

cuits supply energy to the high-side gate drive circuits,

with one for each phase. It is recommended that a

low-ESR ceramic capacitor be connected between the

BST pin and the SW pin of each phase channel (refer

to the “Typical Application Circuit”). The bootstrap

capacitors between the BST and 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 the equation below.

EQUATION 5-36:

5.6

Setting Output Voltage

The MIC2133 requires two resistors to set the output

voltage, as shown in the figure below.

FIGURE 5-3:

Voltage-Divider

Configuration.

PBDDT LS



2IOUT MAX





------------------------------------ V

FBD





tDT



fSW



Where:

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

Body Diode

tDT = Dead Time, which is about 20 ns

PBDQRR LS



VIN MAX



QRR BDLS





fSW



Where:

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

MOSFET Body Diode

PCOSS LS



0.5 COSS LS

 VIN MAX



fSW



Where:

COSS(LS) = Low-Side MOSFET Output Capacitance

PDLS



PCOND LS

 PBDDT LS

 PBDQRR LS

 PCOSS LS



CBSTx

QGHS



V

CBSTx

----------------------

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

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