BD9A300MUV
Charge current to output capacitor ICAP can be expressed in the following method.
(COUT +CLOAD ) ×VOUT
TSS
[ ]
A
ICAP
=
Computation with VIN= 5V, VOUT= 3.3V, L= 1.5µH, switching frequency FOSC= 800kHz(Min), Output capacitor COUT=
44µF, Soft Start time TSS= 0.5ms(Min), load current during soft start IOSS= 2A the method is as below.
(3.8 - IOSS - ΔI /2)×TSS
L
[ ]
- COUT =157.9 μF
CLOAD (max)<
VOUT
If the value of CLOAD is large, and cannot meet the above equation,
(3.8 - IOSS - ΔIL /2)×VFB
CLOAD (max) <
× CSS - COUT
VOUT × ISS
Adjust the value of the capacitor CSS to meet the above formula.
(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.)
Computation with VIN = 5V, VOUT = 3.3V, L = 1.5µH, load current during soft start IOSS = 2A, switching frequency FOSC=
800kHz (Min), Output capacitor COUT = 44µF, VFB = 0.792V(Max), ISS = 3.6µA(Max), A capacitor connected to the CSS if
you want to connect the CLOAD = 220uF is the following equation.
VOUT × ISS
(3.8 - IOSS - ΔIL /2)×VFB
[ ]
× (CLOAD +COUT ) = 2.97 nF
CSS
>
2.
Output Voltage Setting
The output voltage value can be set by the feedback resistance ratio.
VOUT
R1
R2
gm Amp
FB
－
R1 + R2
R2
[ ]
× 0.8 V
＋
VOUT =
0.8V
Figure 51. Feedback Resistor Circuit
3. Soft Start Setting
Turning the EN terminal signal high activates the soft start function. This causes the output voltage to rise gradually
while the current at startup is placed under control. This allows the prevention of output voltage overshoot and inrush
current. The rise time depends on the value of the capacitor connected to the SS terminal.
TSS = (CSS ×VFB )/ISS
TSS : Soft Start Time
CSS : Capacitorconnectedto Soft Start TimeTerminal
VFB : FB TerminalVoltage(0.8V (Typ))
ISS : Soft Start TerminalSource Current (1.8μA(Typ))
withCSS = 0.01μF,
[ ] [ ]
[ ]
TSS = (0.010 μF × 0.8 V )/1.8 μA
[
]
= 4.44 msec
Turning the EN terminal signal high with the SS terminal open (no capacitor connected) or with the terminal signal high
causes the output voltage to rise in 1 msec (Typ).
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TSZ02201-0J3J0AJ00350-1-2
30.Jun.2017 Rev.003
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