B1077AS-4R7N AD [Analog Devices], B1077AS-4R7N Datasheet - Page 17

• Manufacturer Part Number: B1077AS-4R7N
• Description: 1.2 A, 20 V, 700 kHz/1.4 MHz, Nonsynchronous Step-Down Regulator
• Manufacturer: AD [Analog Devices]

Based on three conversion limitations (the minimum on time,
the minimum off time, and the bootstrap dropout
voltage), Figure 44 shows the voltage conversion limitations.
LOW INPUT VOLTAGE CONSIDERATIONS
For low input voltage between 3 V and 5 V, the internal boot
regulator cannot provide enough 5.0 V bootstrap voltage due to
the internal dropout voltage. As a result, the increased MOSFET
R
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 45 shows the application diagram for
the external bootstrap circuit.
DS(ON)
Figure 45. External Bootstrap Circuit for Low Input Voltage Application
reduces the available load current. To prevent this, add
22
17
12
5.5
5.3
5.1
4.9
4.7
4.5
4.3
4.1
3.9
3.7
3.5
7
2
OFF
0
3V ~ 5V
1
Figure 43. Minimum Input Voltage vs. Load Current
ON
Figure 44. Voltage Conversion Limitations
2
VIN
EN
ADP2300/
ADP2301
4
GND
10
LOAD CURRENT (mA)
6
FOR STARTUP
FOR RUNNING
BST
V
SW
FB
MAXIMUM INPUT FOR ADP2300
MAXIMUM INPUT FOR ADP2301
MINIMUM INPUT FOR ADP2300/ADP2301
OUT
8
(V)
SCHOTTKY DIODE
10
100
12
V
f
5V BIAS
VOLTAGE
SW
OUT
= 1.4MHz
14
= 3.3V
16
1k
Rev. 0 | Page 17 of 28
PROGRAMMING THE PRECISION ENABLE
Generally, the EN pin can be easily tied to the VIN pin so that the
device automatically starts up when the input power is applied.
However, the precision enable feature allows the ADP2300/
ADP2301 to be used as a programmable UVLO by connecting
a resistive voltage divider to V
configuration prevents the start-up problems that can occur
when V
load current.
The precision enable feature also allows the ADP2300/ADP2301 to
be sequenced precisely by using a resistive voltage divider with
another dc-to-dc output supply, as shown in Figure 47.
With a 1.2 μA pull-down current on the EN pin, the equation for
the start-up voltage in Figure 46 and Figure 47 is
where:
V
R
R
EN1
EN2
STARTUP
is the resistor from the dc source to EN.
is the resistor from EN to GND.
V
STARTUP
Figure 46. Precision Enable Used as a Programmable UVLO
IN
Figure 47. Precision Enable Used as a Sequencing Control
is the start-up voltage to enable the chip.
ramps up slowly in soft start with a relatively high
OTHER DC-TO-DC
OUTPUT
=
⎛
⎜
⎜
⎝
1
R
2 .
from Another DC-to-DC Output
EN
R
R
V
EN1
EN2
V
2
IN
+
1
2 .
R
μA
EN1
IN
VIN
EN
, as shown in Figure 46. This
ADP2300/
ADP2301
⎞
× ⎟ ⎟
⎠
ADP2300/ADP2301
R
R
EN2
EN
1
EN
+
ADP2300/
ADP2301
1
2 .
V