AN8021L Datasheet, PDF (15/16 Pages) Panasonic Semiconductor – AC-DC switching power supply control IC

Voltage Regulators
AN8021L, AN8021SB
s Application Notes (continued)
[3] Design reference data (continued)
2. Setting the timer latch period
The timer latch period t, the period from the time when an abnormality of the power supply output is detected
to the time when the overvoltage protector is activated, can be set to any desired value by using the external
capacitance C_TIM (C2) based on the following equation:
TIM/OVP = capacitor terminal for timer latch period setting
[Reference calculation formula]
t=
C₂ · V_TIM
I_TIM
[s]
V_TIM = 6 V (typ.): Overvoltage protection threshold value
I_TIM = timer latch charge current
(Varies depending on R7 value, at R7 = 19 kΩ)
I_TIM = 30 µA (typ.)
3. Setting the soft start time
• Soft start charge current
Most of the conventional ICs are charged by using the internal resistor from the internal reference voltage, or
by using the constant current source which is determined by the internal resistance. However, the above charging
method suffers from problems on dispersion or temperature change and can not ensure the soft start time. For this
reason, the AN8021L and AN8021SB use the following method: The soft start charge current is given from the
constant current source used in the internal triangular wave oscillation circuit. In addition, the above constant
current source is stable with respect to dispersion or fluctuation with temperature because it has the current value
which is determined by the external resistor and the terminal voltage given from the resistor-divider of internal
reference voltage. However, for this method, particular care should be taken on the application: Since each time
the setting of oscillation frequency is changed, the soft start constant should be also changed.
SS (C5) = capacitor terminal for soft start
[Reference calculation formula]
t=
C₅ · V_SS
I_SS
[s]
I_SS = soft start charging current
(Varies depending on R7 value, at R_T (R7) = 19 kΩ)
I_SS = 30 µA (typ.)
V_SS = 2.0 V, at duty = 0%
V_SS = 4.1 V, at maximum duty
4. Start circuit
The start time from the power-on to the actual start can be set by using the values of R1 and C1. Too long start
time makes the power supply to rise slowly.
[Setting the start resistor R1]
1) When the overload shutting-off condition is kept, the shut-off bias current (OVP operating bias current) of the
AN8021L and AN8021SB is 550 µA (typical) at V_CC = 10 V. Therefore, set the R1 as shown in the following
equation :
R₁ <
V_IN − 10 V
550 µA
2) When automatic reset is desired after the overload shut-off, the standby current of the AN8021L and AN8021SB
is 70 µA (typical) at V_CC = 12 V. Therefore, set the R1 as shown in the following equation :
V_IN − 10 V
550 µA
< R₁ <
V_IN − 12 V
70 µA
[Setting the C1]
When the AN8021L or AN8021SB is started, the operating supply current of 7.5 mA is required at V_CC = 18 V.
The current should be supplied with the discharge current of the C1 during the period from the soft start time
up to the time when the supply current is supplied from the auxiliary bias coil. Therefore set the C1 as shown in
the following equation:
(V_CC(START) − V_CC(STOP)) · C₁
7.5 mA
> Soft start time
15