FREQ datasheet(33/56 Pages) FUJITSU

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MB39A214A

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FUJITSU SEMICONDUCTOR CONFIDENTIAL

DS405-00007-1v0-E

Selection of Switching FET

If selecting the high-side FET so that the value of the high-side FET conduction loss and the high-side FET

switching loss is same, the loss is effectively decreased.

Confirm that the high-side FET loss is within the rating value.

PMainFET = PRON_Main + PSW_Main

PMainFET

: High-side FET loss [W]

PRON_Main : High-side FET conduction loss [W]

PSW_Main

: High-side FET switching loss [W]

High-side FET conduction loss

VOUT

PRON_Main = IOUT_MAX

VIN

RON_Main

PRON_Main : High-side FET conduction loss [W]

IOUT_MAX : Maximum load current [A]

VIN

: Power supply voltage [V]

VOUT

: Output voltage [V]

RON_Main : ON resistance of high-side FET [Ω]

The high-side FET switching loss can be calculated roughly by the following formula.

PSW_Main

1.56

VIN

fOSC

IOUT_MAX

QSW

PSW_Main : Switching loss [W]

VIN

: Power supply voltage [V]

fOSC

: Switching frequency [Hz]

IOUT_MAX : Maximum load current [A]

QSW

: Amount of high-side FET gate switch electric charge [C]

MOSFET has a tendency where the gate drive loss increases because the lower drive voltage product has

the bigger amount of gate electric charge (QG). Normally, we recommend a 4 V drive product, however, the

idle period at light load (both the high-side FET and the low-side FET is off-period) gets longer and the gate

drive voltage of the high-side FET may decrease, in the automatic PFM/PWM selection mode. The voltage

drops most at no-load mode. At this time, confirm that the boost voltage (voltage between BST-LX pins) is

a big enough value for the gate threshold value voltage of the high-side FET.

If it is not enough, consider adding the boost diode, increasing the capacitor value of the boost capacitor or

using a 2.5 V (or 1.8 V) drive product to the high-side FET.

Select the ON resistance of low-side FET from the range below.

0.2

0.1

0.015

RON_Sync ≤

ΔIL

, RON_Sync ≤

ΔIL

, RON_Sync ≥

ΔIL

(ILIM −

)

RON_Sync

: ON resistance of low-side FET [Ω]

ΔIL

: Ripple current peak-to-peak value of inductor [A]

ILIM

: Over current detection value [A]

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