S-816 Datasheet, PDF (14/24 Pages) Seiko Instruments Inc – EXTERNAL TRANSISTOR TYPE CMOS VOLTAGE REGULATOR

EXTERNAL TRANSISTOR TYPE CMOS VOLTAGE REGULATOR
S-816 Series
Rev.5.1_00
The detection of the overcurrent is done by the sense resistance (R_S) and the PNP transistor (Tr1).
When Tr1 comes on, triggered by a voltage drop of R_S, the NPN transistor (Tr2) also comes on, according
to the time constants of the capacitor (C₂) and resistance (R₂). This causes the shutdown pin to turn to
the "L" level, and the regulating operation to stop, and interrupting the current to the load.
When the load current is cut off, the voltage drop of R_S stops. This makes Tr1 off again, and also makes
the NPN transistor (Tr2) off.
In this condition, the shutdown pin returns to the "H" level, according to the time constants of the
capacitor (C₁) and resistance (R₁). This delay time in which shutdown pin returns to the "H" level from the
"L" level is the time in which the load current remains cut off.
If an overcurrent flows again after the shutdown pin has assumed the "H" level following the delay time
and the regulating operation has been restarted, the circuit will again suspend the regulating operation
and resume the intermittent operation. This intermittent operation will be continued till the overcurrentt is
eliminated, and once theovercurrent disappears, the normal operation will be restored.
The overcurrent detection value (I_{OUT_MAX}) is represented by the following equation:
I_{OUT_MAX} = |V_BE1| / R_S
In this case, R_S denotes the resistance value of the sense resistance, and V_BE1 denotes the base-emitter
saturation voltage of Tr1.
For the PNP transistor (Tr1) and the NPN transistor (Tr2), try to select those of small-signal type that offer
a sufficient withstand voltage against the input voltage (V_IN).
The on-time (t_ON) and the off-time (t_OFF) of the intermittent operation are broadly expressed by the
following equations:
t_ON = −1 × C₂ × R₂ × L_N ( 1 − ( V_BE2 × ( 1 + R₂ / R₃ ) ) / ( V_IN − V_BE1 ) )
t_OFF = −1 × C₁ × R₁ × L_N ( 1 − V_SH / V_IN )
In this case, V_BE2 denotes the base-emitter saturation voltage of Tr2, V_IN denotes the input voltage, and
V_SH denotes the inversion voltage ("L"→"H") of the shutdown pin.
Set the on-time value that does not cause the overcurrent protection to be activated by a rush current to
the load capacitor. Then, compute the ratio between the on-time and the off-time from the maximum
input voltage of the appropriate application and the power dissipation of the external PNP transistor, and
decide the off-time with reference to the on-time established earlier.
Take the equation above as a rough guide, because the actual on-time (t_ON) and off-time (t_OFF) should be
defined and checked using the utilizing components.
2. External Adjustment of Output Voltage
The S-816 Series allows you to adjust the output voltage or to set its value over the output voltage range
(6 V) of the products of this series, when external resistances R_A, R_B and capacitor C_C are added, as
illustrated in Figure 13. Moreover, a temperature gradient can be obtained by inserting a thermistor or
other element in series with external resistances R_A and R_B.
OUT
VIN
+
V_{IN −} C_IN
ON/ OFF
EXT
C_C
R_A
VOUT
Error
R₁
amplifier ₊
R_B
−
R₂
VREF
+
− ^CL
VSS
Figure 13
14
Seiko Instruments Inc.