ADP1621ARMZ-R7 datasheet(16/32 Pages) AD

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ADP1621

Rev. A | Page 16 of 32

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 tem-

perature, and 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

( K

DSON

LOAD

⎟

⎠

⎞

⎜

⎝

⎛

−

)

(19)

where PC is the conduction power loss, and 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, and 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.

The total power dissipation also determines the MOSFET

junction temperature, which is given by

MOSFET

(23)

where TJ,MOSFET is the junction temperature, TA is the ambient

temperature, and θJA is the junction-to-ambient thermal

resistance of the MOSFET package. The MOSFET junction

temperature must not exceed its maximum rating at the given

power dissipation level.

If lossless current sensing is not used, there will also be power

dissipation in the external current-sense resistor, RCS. The power

dissipation, PCS, in the external resistor due to conduction losses

is given by

LOAD

⎟

⎠

⎞

⎜

⎝

⎛

−

(24)

LOOP COMPENSATION

The ADP1621 uses external components to compensate the

regulator loop, allowing optimization of the loop dynamics for

a given application.

The step-up converter produces an undesirable right-half plane

(RHP) zero in the regulation feedback loop. This RHP zero

requires compensating the regulator such that the crossover

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