BD9B500MUV Datasheet, PDF (22/34 Pages) Rohm – Integrated MOSFET Single Synchronous Buck DC/DC Converter

BD9B500MUV
Charge current to output capacitor ICAP can be expressed using the following equation.
I_CAP
=
(C_OUT
+ C _LOAD ) ×V_OUT
T_SS
[_A]
For example, given VIN= 5V, VOUT= 3.3V, L= 1.5µH, switching frequency fSW= 1.2MHz(Max), Output capacitor
COUT=44µF, Soft Start time TSS=0.5ms(Min), and load current during soft start IOSS= 5A, maximum CLOAD can be
computed using the following equation.
C _LOAD(max)<
(5.5
-
I_OSS
 ΔI _L /2)× T_SS
V_OUT
- C_OUT
78.9
μF
If the value of CLOAD is large, and cannot meet the above equation, adjust the value of the capacitor CSS to meet the
condition below.
C _LOAD(max) <
(5.5
- I_OSS  ΔI _L /2)×V_FB
V_OUT × I _SS
× C _SS
- C_OUT
(Refer to the following items (3) Soft Start Setting equation of time TSS and soft-start value of the capacitor to be
connected to the CSS.)
For example, given VIN = 5V, VOUT = 3.3V, L = 1.5µH, load current during soft start IOSS = 5A, switching frequency
fSW=1.2MHz (Max), Output capacitor COUT = 44µF, VFB = 0.792V(Max), ISS = 2.0µA(Max), with CLOAD = 220uF, capacitor
CSS is computed as follows.
C _SS
>
V_OUT × I _SS
(5.5 - I_OSS  ΔI _L /2)×V_FB
× (C _LOAD + C_OUT ) = 2710
pF
※CLOAD has an effect on the stability of the DC/DC converter.
To ensure the stability of the DC/DC converter, make sure that a sufficient phase margin is provided.
2. Output Voltage Setting
The output voltage value can be set by the feedback resistance ratio.
For stable operation, it is recommended to use feedback resistance R1 of more than 20kΩ.
V_OUT
R1
Error Amplifier
VOUT
=
R1 + R2
R2
×
0.8
V
FB
R2
0.8V
R2 = ^0.8 × R1 Ω
VOUT - 0.8
Figure 51. Feedback Resistor Circuit
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TSZ02201-0J3J0AJ01280-1-2
25.May.2016 Rev.001