B1077AS-4R7N AD [Analog Devices], B1077AS-4R7N Datasheet - Page 21

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

DESIGN EXAMPLE
This section provides the procedures to select the external com-
ponents, based on the example specifications listed in Table 10.
The schematic for this design example is shown in Figure 48.
Table 10. Step-Down DC-to-DC Regulator Requirements
Parameter
Input Voltage, V
Output Voltage, V
Programmable
SWITCHING FREQUENCY SELECTION
Select the switching frequency—700 kHz (ADP2300) or 1.4 MHz
(ADP2301)—using the conversion limitation curve shown
in Figure 44 to assess the conversion limitations (the minimum o
time, the minimum off time, and the bootstrap dropout voltage).
For example, in Figure 44 V
sion limitation for both the 700 kHz and 1.4 MHz switching
frequencies for an output voltage of 3.3 V, but choosing the 1.4 MHz
switching frequency provides the smallest sized solution. If higher
efficiency is required, choose the 700 kHz option; however, the
PCB footprint area of the regulator will be larger because of the
bigger inductor and output capacitors.
CATCH DIODE SELECTION
Select the catch diode. A Schottky diode is recommended for best
efficiency because it has a low forward voltage drop and faster
switching speed. The average current of the catch diode in
normal operation, with a typical Schottky diode forward
voltage, can be calculated using the following equation:
where:
V
V
I
V
Therefore, I
However, for the worst-case condition, in which there is a shorted
output, the diode current would be increased to 2 A typical, deter-
mined by the peak switch current limit (see Table 1). In this case,
selecting a B230A, 2.0 A/30 V surface-mount Schottky diode
would result in more reliable operation.
LOAD(max)
OUT
IN
D
UVLO Voltage
= 0.4 V.
= 12 V.
I
= 3.3 V.
DIODE
= 1.2 A.
(
DIODE(AVG)
AVG
)
IN
OUT
=
⎛
⎜
⎜
⎝
1
= 0.85 A.
−
Specification
12.0 V ± 10%
3.3 V, 1.2 A, 1% V
ripple at CCM mode
V
approximately 7.8 V
V
V
IN
OUT
IN
start-up voltage
IN
+
+
= 12 V ± 10% is within the conver-
V
V
D
D
⎞
⎟
× ⎟
⎠
I
LOAD
OUT
(max)
Additional
Requirements
None
None
None
Rev. 0 | Page 21 of 28
n
INDUCTOR SELECTION
Select the inductor by using the following equation:
where:
V
V
I
V
f
This results in L = 5.15 μH. The closest standard value is 4.7 μH;
therefore, ΔI
The inductor peak current is calculated using the following
equation:
where:
I
ΔI
Therefore, the calculated peak current for the inductor is 1.397 A.
However, to protect the inductor from reaching its saturation
point in the current-limit condition, the inductor should be rated
for at least a 2.0 A saturation current for reliable operation.
OUTPUT CAPACITOR SELECTION
Select the output capacitor based on the output voltage ripple
requirement, according to the following equation:
where:
ΔI
f
ΔV
If the ESR of the ceramic capacitor is 3 mΩ, then C
Because the output capacitor is one of the two external components
that control the loop stability, most applications using the ADP2301
(1.4 MHz switching frequency) require a minimum 10 μF capaci-
tance to ensure stability. According to the recommended external
components in Table 11, choose 22 μF with a 6.3 V voltage rating
for this example.
SW
SW
LOAD(max)
LOAD(max)
OUT
IN
D
RIPPLE
RIPPLE
= 0.4 V.
= 1.4 MHz.
= 1.4 MHz.
RIPPLE
= 12 V.
L
= 3.3 V.
I
Δ
PEAK
V
=
= 0.394 A.
= 0.394 A.
= 33 mV.
RIPPLE
= 1.2 A.
= 1.2 A.
0
3 .
=
×
RIPPLE
(
I
V
I
=
LOAD
IN
LOAD
Δ
= 0.394 A.
−
I
(max)
RIPPLE
V
(max)
OUT
+
×
×
)
Δ
f
⎛
⎜
⎜
⎝
I
sw
8
RIPPLE
2
×
×
⎛
⎜
⎜
⎝
f
sw
V
V
ADP2300/ADP2301
1
OUT
×
IN
C
+
OUT
+
V
V
D
D
+
⎞
⎟
⎟
⎠
ESR
C
OUT
OUT
⎞
⎟
⎟
⎠
= 1.2 μF.