ADP1621ARMZ-R7 datasheet(16/32 Pages) AD

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ADP1621

Data Sheet

Rev. D | Page 16 of 32

DIODE SELECTION

The diode conducts the inductor current to the output capacitor

and load while the MOSFET is off. The average diode current is

the load current:

LOAD

AVE

DIODE

(14)

The rms diode current in continuous conduction mode is given by

LOAD

RMS

DIODE

−

(15)

where D is the duty cycle.

The power dissipated in the diode is

LOAD

DIODE

(16)

where VD is the forward-voltage drop of the diode.

The total power dissipation determines the diode junction

temperature, which is given by

DIODE

(17)

where:

TJ,DIODE is the junction temperature.

TA is the ambient temperature.

θJA is the junction-to-ambient thermal resistance of the diode

package. The diode junction temperature must not exceed its

maximum rating at the given power dissipation level.

For high efficiency, Schottky diodes are recommended. The low

forward-voltage drop of a Schottky diode reduces the power losses

during the MOSFET off time, and the fast switching speed reduces

the switching losses during the MOSFET transitions. However,

for high voltage, high temperature applications where the reverse

leakage current of the Schottky diode can become significant

and degrade efficiency, use an ultrafast-recovery junction diode.

Make sure that the diode is rated to handle the average output

load current. Many diode manufacturers derate the current

capability of the diode as a function of the duty cycle. Verify

that the diode is rated to handle the average output load current

with the minimum duty cycle. Also, ensure that the peak inductor

current is less than the maximum rated current of the diode.

MOSFET SELECTION

When turned on, the external n-channel MOSFET allows

energy to be stored in the magnetic field of the inductor. When

the MOSFET is turned off, this energy is delivered to the load to

boost the output voltage.

The choice of the external power MOSFET directly affects the

boost converter performance. Choose the MOSFET based on

the following: threshold voltage (VT), on resistance (RDSON),

maximum voltage and current ratings, and gate charge.

The minimum operating voltage of the ADP1621 is 2.9 V.

Choose a MOSFET with a VT that is at least 0.3 V less than the

minimum input supply voltage at PIN used in the application.

Ensure that the maximum VGS rating of the MOSFET is at least

a few volts greater than the maximum voltage that is applied to

PIN. Ensure that the maximum VDS rating of the MOSFET

exceeds the maximum VOUT by at least 5 V to 10 V. Depending

on parasitics, the MOSFET may be exposed to voltage spikes that

exceed the sum of VOUT and the forward-voltage drop of the diode.

Estimate the rms current in the MOSFET under continuous

conduction mode by

LOAD

RMS

MOSFET

−

(18)

where D is the duty cycle. Derate the MOSFET current at least

20% to account for inductor ripple and changes in the forward-

voltage drop of the diode.

The MOSFET power dissipation due to conduction is thus

(

)

DSON

LOAD













−

(19)

where:

PC is the conduction power loss.

RDSON is the MOSFET on resistance. The variable K is a factor

that models the increase of RDSON with temperature:

(

)

005



−

J,MOSFET

(20)

where TJ,MOSFET is the MOSFET junction temperature. Note that

multiple n-channel MOSFETs can be placed in parallel to reduce

the effective RDSON.

The power dissipation due to switching transition loss is

approximated by

(

)

(

)

LOAD

OUT

−

(21)

where:

PSW is the switching power loss.

tR is the MOSFET rise time.

tF is the MOSFET fall time. The MOSFET rise and fall times are

functions of both the gate drive circuitry and the MOSFET used

in the application.

The total power dissipation of the MOSFET is the sum of the

conduction and transition losses:

MOSFET

(22)

where PMOSFET is the total MOSFET power dissipation. Ensure

that the maximum power dissipation is significantly less than

the maximum power rating of the MOSFET.

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