ADP1621ARMZ-R7(RevB) 查看數據表（PDF） - Analog Devices
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ADP1621ARMZ-R7 Datasheet PDF : 32 Pages
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ADP1621
The total power dissipation determines the diode junction
temperature, which is given by
TJ ,DIODE = TA + PDIODE × θ JA
(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
I MOSFET ,RMS
=
I LOAD
1−D
×
D
(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.
Data Sheet
The MOSFET power dissipation due to conduction is thus
PC
=
I LOAD
1− D
2
× D × RDSON
× (1+ K )
(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:
( ) K = 0.005/ C × TJ,MOSFET − 25C
(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
( ) ( ) PSW
=
VOUT + VD
× I LOAD ×
1−D
2
tR +tF
× fSW
(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:
PMOSFET = PC + PSW
(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
TJ ,MOSFET = TA + PMOSFET × θ JA
(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
PCS
=
I LOAD
1− D
2
× D × RCS
(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
Rev. B | Page 16 of 32
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