Datasheet

Data Sheet ADP2302/ADP2303
Rev. A | Page 17 of 28
2
4
6
8
10
12
14
16
18
20
22
0246810121416
V
IN
(V)
V
OUT
(V)
08833-147
MAXIMUM INPUT VOLTAGE
MINIMUM INPUT VOLTAGE
Figure 47. Voltage Conversion Limitations
LOW INPUT VOLTAGE CONSIDERATIONS
For low input voltage between 3 V and 5 V, the internal boot
regulator cannot provide enough bootstrap voltage due to the
internal dropout voltage. As a result, the increased MOSFET
R
DS(ON)
reduces the available load current. To prevent this, add
an external small-signal Schottky diode from a 5.0 V external
bootstrap bias voltage. Because the absolute maximum rating
between the BST and SW pins is 6.0 V, the bias voltage should
be less than 5.5 V. Figure 48 shows the application diagram for
the external bootstrap circuit.
VIN
3
.0V ~ 5.0
V
ADP2302/
ADP2303
EN
GND
O
FF
ON
BST
SW
SCHOTTKY
DIODE
5V BIAS VOLTAGE
FB
08833-046
Figure 48. External Bootstrap Circuit for Low Input Voltage Application
PROGRAMMING THE PRECISION ENABLE
Generally, the EN pin can connect to the VIN pin so that the
device automatically starts up when the input power is applied.
However, the precision enabling feature allows the ADP2302/
ADP2303 to be used as a programmable UVLO by connecting
a resistive voltage divider to VIN, as shown in Figure 49. This
configuration prevents the start-up problems that can occur
when VIN ramps up slowly in soft start with a relatively high
load current.
VIN
V
IN
R
EN1
R
EN2
ADP2302/
ADP2303
EN
0
8833-047
Figure 49. Precision Enable Used as a Programmable UVLO
The precision enable feature also allows the ADP2302/ADP2303 to
be sequenced precisely by using a resistive voltage divider from
another dc-to-dc power supply, as shown in Figure 50.
R
EN1
R
EN2
ADP2302/
ADP2303
EN
ANOTHER
DC/DC
SUPPLIER
0
8833-048
Figure 50. Precision Enable Used as a Sequencing Control
from Another DC-to-DC Power Supply
With a 1.2 µA pull-down current on the EN pin, the equation for
the start-up voltage in Figure 49 and Figure 50 is
V2.1A2.1
V2.1










EN1
EN2
STARTUP
R
R
V
where:
V
STARTUP
is the start-up voltage to enable the chip.
R
EN1
is the resistor from the dc source to EN.
R
EN2
is the resistor from EN to GND.
INDUCTOR
The high switching frequency of the ADP2302/ADP2303 allows
the use of small inductors. For best performance, use inductor
values between 1 H and 15 H.
The peak-to-peak inductor ripple current is calculated using the
following equation:














DIN
D
OUT
sw
OUT
IN
RIPPLE
VV
VV
fL
VV
I
)(
where:
f
SW
is the switching frequency.
L is the inductor value.
V
D
is the diode forward drop.
V
IN
is the input voltage.
V
OUT
is the output voltage.
Inductors of smaller values are usually smaller in size but
increase the ripple current and the output ripple voltage. As a
guideline, the inductor peak-to-peak ripple current is typically
set to 30% of the maximum load current for optimal transient