S816 datasheet(14/24 Pages) SII

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EXTERNAL TRANSISTOR TYPE CMOS VOLTAGE REGULATOR

S-816 Series

Rev.6.1_00

Seiko Instruments Inc.

The detection of the overcurrent is done by the sense resistance (RS) and the PNP transistor (Tr1).

When Tr1 comes on, triggered by a voltage drop of RS, the NPN transistor (Tr2) also comes on, according

to the time constants of the capacitor (C2) and resistance (R2). This causes the ON/OFF pin to turn to

OFF 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 RS stops. This makes Tr1 off again, and also makes

the NPN transistor (Tr2) off.

In this condition, the ON/OFF pin returns to ON level, according to the time constants of the capacitor (C1)

and resistance (R1). This delay time in which ON/OFF pin returns to ON level from OFF level is the time

in which the load current remains cut off.

If an overcurrent flows again after the ON/OFF pin has assumed ON 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 the overcurrent disappears, the normal operation will be restored.

The overcurrent detection value (IOUT_MAX) is represented by the following equation:

IOUT_MAX

= |VBE1| / RS

In this case, RS denotes the resistance value of the sense resistance, and VBE1 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 (VIN).

The on-time (tON) and the off-time (tOFF) of the intermittent operation are broadly expressed by the

following equations:

tON

= −1 × C2 × R2 × LN ( 1 − ( VBE2 × ( 1 + R2 / R3 ) ) / ( VIN − VBE1 ) )

tOFF

= −1 × C1 × R1 × LN ( 1 − VSH / VIN )

In this case, VBE2 denotes the base-emitter saturation voltage of Tr2, VIN denotes the input voltage, and

VSH denotes the inversion voltage ("L"

→"H") of the

OFF

ON/

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 (tON) and off-time (tOFF) 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 RA, RB and capacitor CC 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 RA and RB.

VIN

CIN

−

ON/OFF

VIN

EXT

−

VOUT

VSS

OUT

−

Error

amplifier

Vref

Figure 13

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