SSC2005SC datasheet(13/19 Pages) SANKEN

13 / 19 page

SSC2005SC

SSC2005SC-DS Rev.1.1

SANKEN ELECTRIC CO.,LTD.

Feb. 06, 2015

9.2

External Components

Take care to use properly rated, including derating as

necessary and proper type of components.

Figure 9-1 shows the IC peripheral circuit.

VOSC

VSET

OSC

Error AMP

PWM COMP

VFB

= 2.5V

VOUT

RVS1

RVS2

GND

COMP

VCS(ZCD)= -10mV

OUT

RCS

RDLY

RRT

RDLY

DZCS

ZCD COMP

VCOMP

Figure 9-1 The IC peripheral circuit.

 FB Pin Peripheral Circuit (Output VoltageDetection)

The output voltage VOUT is set using RVS1 and RVS2.

It is expressed by the following equation:

OUT





















(V)

(6)

where,

VFB

: Feedback reference voltage = 2.50 V

IFB

: Bias current = − 2.0 µA

RVS, RVS2

: Combined resistance to set VOUT (Ω)

Since RVS1 have applied high voltage and have high

resistance value, RVS1 should be selected from

resistors designed against electromigration or use a

combination of resistors for that.

The value of capacitor C6 between the FB pin and the

GND pin is set approximately 100 pF to 3300 pF, in

order to reduce the switching noise.

 COMP Pin Peripheral Circuit: R

S, CS and CP

Figure 9-1 shows the IC peripheral circuit.

The FB pin voltage is induced into internal Error AMP.

The output voltage of the Error AMP is averaged by the

COMP pin. The on-time control is achieved by

comparing the signal VCOMP and the ramp signal VOSC.

CS and RS adjust the response speed of changing on-time

according to output power.

The typical value of CS and RS are 1 μF and 10 kΩ,

respectively. When CS value is too large, the response

becomes slow at dynamic variation of output and the

output voltage decreases.

Since CS and RS affect on the soft-start period at startup,

adjustment is necessary in actual operation.

The ripple of output detection signal is averaged by CP.

When the CP value is too small, the IC operation may

become unstable due to the output ripple. The value of

capacitor CP is approximately 0.47 μF.

 RT Pin Peripheral Circuit: R

RT and C3

The value of capacitor C3 in parallel with RRT is

approximately 0.01 μF, in order to reduce the switching

noise.

RRT is 15 kΩ to 47 kΩ for the adjustment of maximum

on-time, tON(MAX).

Refer to Section 8.4 for RRT setting.

 RDLY Pin Peripheral Circuit: R

DLY and C4

RDLY shown in Figure 9-1 is for the adjustment of the

turn-on delay time, tDLY, of the Power MOSFET. As

shown in Section 8.5 Zero Current Detection, adjust the

value of RDLY and turn-on timing to the bottom point of

VDS free oscillation waveform on actual operation in the

application. The range of RDLY is 15 kΩ to 56 kΩ.

The value of capacitor C4 is approximately 0.01 μF, in

order to reduce the switching noise.

 CS Pin Peripheral Circuit: R

CS, R4, C5 and DZCS

RCS shown in Figure 9-1 is current sensing resistor. RCS

is calculated by the following Equation (7), where

Overcurrent Protection Threshold Voltage VCS(OCP) is −

0.60 V and ILP is calculated by Equation (1).

)

OCP

(



(Ω)

(7)

The CR filter (R4 and C5) prevents IC from

responding to

the drain current surge at power

MOSFET turn-on and avoids the unstable operation

of the IC.

R4 value of approximately 47 Ω is recommended,

since the CS Pin Source Current affects the accuracy

of OCP detection (see Section 8.5).

C5 value is recommended to be calculated by using

following equation in which the cut-off frequency of

the CR filter (C5 and R4) is approximately 1 MHz.





(F)

(8)

If R4 value is 47 Ω, C5 value is approximately

3300 pF.

The absolute voltage of the CS pin is −5 V. The CS

pin voltage may exceed the absolute value when the

startup current to a charge output capacitor, C2, flows

RCS. Thus, DZCS is used for the overvoltage protection

of the CS pin.

DZCS value of approximately 3.9 V is recommended.

The value should be higher than VCS(OCP) and be lower

than the CS pin absolute maximum rating of −5 V.

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