Datasheet

EVAL-ADP1621
Rev. 0 | Page 3 of 12
EVALUATION BOARD HARDWARE
Table 1. Jumper Description
Jumper Description Function
J1
V
IN
and GND
terminals
J2
V
OUT
and GND
terminals
J3 SDSN connection
Connects SDSN to V
IN
or drives
SDSN separately
Table 2. Demo Board Specifications
Parameter Description
V
IN
Input range , 3.0 V to 3.6 V
V
OUT
5 V
f
SW
Switching frequency, 600 kHz
Efficiency 90% at 1 A load
Maximum Load 2 A
Duty Cycle 40% (V
IN
= 3.3 V)
Output Ripple 30 mV with 1 A load at 25°C
Input Ripple 25 mV at 25°C
COMPONENT SELECTION
For more information about component selection, refer to the
ADP1621 data sheet available. Consult the ADP1621 data sheet
in conjunction with this data sheet when using the evaluation
board.
Duty Cycle
D
OUT
IND
OUT
VV
VVV
D
+
−+
=
(1)
where:
V
OUT
is the desired output voltage.
V
IN
is the input voltage.
V
D
is the forward-voltage drop of the diode.
With V
IN
= 3.3 V, and V
OUT
= 5 V, the duty cycle is calculated to
be about 40%, assuming V
D
is about 0.5 V.
Feedback Resistors
⎟
⎠
⎞
⎜
⎝
⎛
+×=
R2
R1
V
OUT
1V215.1 (2)
With V
OUT
= 5 V and if R2 = 5.6 kΩ, then R1 is calculated to be
17.4 kΩ.
Inductor Selection
The average inductor current, I
L,AVE
, is given by
D
I
I
LOAD
AVEL
−
=
1
,
(3)
where I
LOAD
is the load current.
With I
LOAD
= 2 A and D = 0.4, I
L,AVE
is calculated to be 3.33 A. To
reduce the size of the inductor and to minimize the output bulk
capacitance, set the switching frequency f
SW
to 600 kHz.
Assuming a 30% ripple current of 1/(1 − D) × 2 A (the load
current) in the inductor, the inductor value is calculated from
the following equation:
(
)
MAXLOADSW
IN
If
DDV
L
,
3.0
1
××
−
×
×
=
(4)
L is then calculated to be 2.2 µH. For this demo board, a 2.5 µH
is selected. Make sure that the selected inductor can handle the
average dc current of 3.33 A and a peak current of at least 4 A,
which is given by
Lf
DV
D
I
I
D
I
I
SW
INLOADLLOAD
PKL
××
×
+
−
=
Δ
+
−
=
2121
,
(5)
Input Capacitor Selection
Because ceramic capacitors have very low ESR. (a few m), a
47 µF, 6.3 V Murata GRM31CR60J476M (size 1206) ceramic
capacitor is chosen for the input capacitor. If the load current
gets larger than 2 A, a larger input capacitor is needed to reduce
the input ripples current. Other types of input capacitors with
higher ESR can be used, such as the aluminum electrolytic or
the aluminum polymer types. Make sure the ripple current
rating of the input capacitor, I
CIN,RMS
, is greater than
2
3
1
,
L
RMSCIN
I
I
Δ
×= (6)
where I
L
is the peak-to-peak inductor ripple current.
Output Capacitor Selection
The output voltage ripple for a given C
OUT
, ESR, and ESL, the
effective equivalent series inductance of C
OUT
, can be found by
solving the following equation:
×
⎟
⎠
⎞
⎜
⎝
⎛
Δ
+
−
≈Δ
21
L
LOAD
OUT
I
D
I
V
()
22
2
2
2
1
ESLfESR
Cf
SW
OUT
SW
××π++
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××π
(7)
By setting an output voltage ripple equal to 1% (50 mV) of the
output voltage, Equation 7 yields that the minimum C
OUT
required is 20 µF by using an ESR of 2 mΩ and an ESL of
100 nH. The effective capacitance of a ceramic capacitor
generally decreases with increasing bias voltage and with
smaller casings in the same bias voltage. For the demo board,
four 10 µF/16 V (size 1210) ceramic capacitors were selected.
Other combinations of capacitance and ESR are possible by
choosing a much larger C
OUT
with a larger ESR.