Datasheet

EVAL-ADP1621
Rev. 0 | Page 4 of 12
Diode Selection
The diode conducts the inductor current to the output capacitor
and loads while the MOSFET is off. The average diode current
is the load current,
LOADAVEDIODE
II =
,
(8)
The rms diode current in continuous conduction mode is given by
D
D
I
I
LOAD
RMSDIODE
−×
−
= 1
1
,
(9)
where
D is the duty cycle.
With a 2 A load, I
DIODE,RMS
is calculated to be 2.6 A. A Vishay
SSA33L in a SMA package meets the dc current and thermal
requirements.
MOSFET Selection
The power MOSFET must be chosen based on threshold
voltage (V
T
), on resistance (R
DSON
), maximum voltage and
current ratings, and gate charge. The RMS current through the
MOSFET is given by the following equation:
D
D
I
I
LOAD
MOSFETRMS
×
−
=
1
,
(10)
where I
RMS,MOSFET
is calculated to be 2.1 A, assuming I
LOAD
is 2 A
and D is 0.4.
The IRF7470 is a 40 V n-channel power MOSFET that meets
the current and thermal requirements. It comes in an 8-lead
SOIC package and offers low R
DSON
and gate charge. R
DSON
is
30 mΩ at V
GS
= 2.8 V and 15 mΩ at V
GS
= 4.5 V. This general-
purpose MOSFET meets a wide range of output voltage and
current requirements. An alternative to this part is the Vishay
Si7883DP, a 20 V n-channel power MOSFET.
Loop Compensation
The location of the right hand plane (RHP) zero frequency is
determined by the following equation:
()
L
R
Df
LOAD
RHPZ
×π
×−=
2
1
2
,
(11)
where:
f
Z,RHP
is the RHP zero frequency.
R
LOAD
is the equivalent load resistance, or the output voltage
divided by the load current.
To stabilize the regulator, ensure that the regulator crossover
frequency is ≤1/5th of the RHP zero frequency and ≤1/15th of
the switching frequency. With V
OUT
= 5 V at 2 A load, f
Z,RHP
is
calculated to be 57.3 kHz.
For an initial practical design, set the crossover frequency f
C
to
the lower frequency of
15
SW
C
f
f =
(12)
and
5
,RHPZ
f
C
f = (13)
where:
f
C
is the crossover frequency.
f
SW
is the switching frequency.
Equation 13 shows that f
C
is 40 kHz and is lower frequency than
that of Equation 12. The loop compensation components are
calculated to be R
COMP
= 33 kΩ and C
COMP
= 490 pF from Equation
14 and Equation 15, respectively. The final component values need
to be tested and verified on the actual PCB.
R
COMP
is given by
()
m
FB
OUT
CS
OUT
C
COMP
gDV
VRnCf
R
×−×
×××××π
=
1
2
(14)
where:
V
FB
= 1.215 V
D = 0.4
g
m
= 300 μS
R
CS
= 15 mΩ for the IRF7470 at V
GS
= 4.5 V
V
OUT
= 5 V
n = 9.5 (typically)
C
OUT
≈ 40 μF
f
C
= 40 kHz (f
SW
/15 = 60 kHz/15).
To fine-tune the R
COMP
and C
COMP
values on the evaluation
board, run a step load, for example, from 0.2 A to 1 A, at the
output and observe the output transient. If there is too much
overshoot in the transient, increase R
COMP
; if there is too much
oscillation, increase C
COMP
.
Once the compensation resistor R
COMP
is known, set the zero
formed by the resistor and compensation capacitor C
COMP
to
1/4th of the crossover frequency, or
COMPC
COMP
Rf
C
××π
=
2
(15)
A roll-off capacitor of C2 = 390 pF is also added on the demo
board. A smaller C2 works fine.
Slope Compensation
The slope-compensation resistor R
S
= 142 Ω from the following
equation:
(
)
(
)
LfI
ftVVVR
R
SWPKSC
SWMINOFF
IND
OUT
CS
S
×××
×
−×
−
+
×
>
,
,
2
1
(16)
where:
t
OFF,MIN
= 230 ns
f
SW
= 600 kHz
L = 2.5 μH
I
SC,PK
= 70 μA.