MIC2103 Datasheet, PDF(23/36 Page) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

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MIC2103 Datasheet, PDF (23/36 Pages) Micrel Semiconductor – 75V, Synchronous Buck Controllers featuring Adaptive On-Time Control

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Micrel, Inc.

gate-drive current for the low-side MOSFET is more

accurately calculated using CISS at VDS = 0 instead of

gate charge.

For the low-side MOSFET:

IG[low-side] (avg) ï½ CISS ï´ VGS ï´ fSW

(Eq. 6)

Since the current from the gate drive comes from the

VDD, the power dissipated in the MIC2103/04 due to gate

drive is:

(Eq. 7)

PGATEDRIVE ï½ VDD ï´ (IG[high-side] (avg) ï« IG[low-side] (avg))

A convenient figure of merit for switching MOSFETs is

the on resistance multiplied by the total gate charge;

RDS(ON) Ã QG. Lower numbers translate into higher

efficiency. Low gate-charge logic-level MOSFETs are a

good choice for use with the MIC2103/04. Also, the

RDS(ON) of the low-side MOSFET will determine the

current-limit value. Please refer to âCurrent Limitâ

subsection is Functional Description for more details.

Parameters that are important to MOSFET switch

selection are:

ï· Voltage rating

ï· On-resistance

ï· Total gate charge

The voltage ratings for the high-side and low-side

MOSFETs are essentially equal to the power stage input

voltage VHSD. A safety factor of 20% should be added to

the VDS(max) of the MOSFETs to account for voltage

spikes due to circuit parasitic elements.

The power dissipated in the MOSFETs is the sum of the

conduction losses during the on-time (PCONDUCTION) and

the switching losses during the period of time when the

MOSFETs turn on and off (PAC).

PSW ï½ PCONDUCTION ï« PAC

PCONDUCTION ï½ ISW(RMS)2 ï´ RDS(ON)

PAC ï½ PAC(off ) ï« PAC(on)

(Eq. 8)

(Eq. 9)

(Eq. 10)

where:

RDS(ON) = On-resistance of the MOSFET switch

D = Duty Cycle = VOUT / VHSD

Making the assumption that the turn-on and turn-off

MIC2103/04

transition times are equal; the transition times can be

approximated by:

ï½

CISS

ï´ VIN

ï« COSS

ï´ VHSD

(Eq. 11)

where:

CISS and COSS are measured at VDS = 0

IG = Gate-drive current

The total high-side MOSFET switching loss is:

PAC ï½ (VHSD ï« VD ) ï´ IPK ï´ t T ï´ fSW

(Eq. 12)

where:

tT = Switching transition time

VD = Body diode drop (0.5V)

fSW = Switching Frequency

The high-side MOSFET switching losses increase with

the switching frequency and the input voltage VHSD. The

low-side MOSFET switching losses are negligible and

can be ignored for these calculations.

Inductor Selection

Values for inductance, peak, and RMS currents are

required to select the output inductor. The input and

output voltages and the inductance value determine the

peak-to-peak inductor ripple current. Generally, higher

inductance values are used with higher input voltages.

Larger peak-to-peak ripple currents will increase the

power dissipation in the inductor and MOSFETs. Larger

output ripple currents will also require more output

capacitance to smooth out the larger ripple current.

Smaller peak-to-peak ripple currents require a larger

inductance value and therefore a larger and more

expensive inductor.

A good compromise between size, loss and cost is to set

the inductor ripple current to be equal to 20% of the

maximum output current.

The inductance value is calculated by Equation 13:

ï½

VOUT ï´ (VIN(max) ï VOUT )

VIN(max) ï´ fsw ï´ 20% ï´ IOUT(max)

(Eq. 13)

where:

August 2012

M9999-080712-A

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