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STR4A164D Datasheet(PDF) 10 Page - Sanken electric |
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STR4A164D Datasheet(HTML) 10 Page - Sanken electric |
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10 / 23 page  STR4A100 Series STR4A100 - DSJ Rev.3.3 SANKEN ELECTRIC CO.,LTD. 10 Aug.06, 2015 http://www.sanken-ele.co.jp © SANKEN ELECTRIC CO.,LTD. 2011 10. Operational Description All of the parameter values used in these descriptions are typical values, unless they are specified as minimum or maximum. With regard to current direction, "+" indicates sink current (toward the IC) and "–" indicates source current (from the IC). 10.1 Startup Operation Figure 10-1 shows the circuit around the VCC pin. The IC incorporates the startup circuit. The circuit is connected to D/ST pin. When the D/ST pin voltage reaches to Startup Circuit Operation Voltage VSTARTUP = 29 V, the startup circuit starts operation. During the startup process, the constant current, ISTARTUP = − 2.1 mA, charges C2 at the VCC pin. When VCC pin voltage increases to VCC(ON) = 15.2 V, the control circuit starts switching operation. During the IC operation, the voltage rectified the auxiliary winding voltage, VD, of Figure 10-1 becomes a power source to the VCC pin. After switching operation begins, the startup circuit turns off automatically so that its current consumption becomes zero. VAC C1 D2 R2 C2 T1 D P D1 VCC S/GND D/ST 4 5~8 2 VD U1 Figure 10-1 VCC pin peripheral circuit The approximate value of auxiliary winding voltage is 15 to 20 V, taking account of the winding turns of D winding so that the VCC pin voltage becomes Equation (1) within the specification of input and output voltage variation of power supply. .) (min V V .) (max V ) OVP ( CC CC ) BIAS ( CC ⇒10.5 (V) < V CC < 27.5 (V) (1) The startup time of the IC is determined by C2 capacitor value. The approximate startup time tSTART is calculated as follows: ) ST ( CC ) INT ( CC ) ON ( CC START I V V × C2 t － (2) Where, tSTART : Startup time of the IC (s) VCC(INT) : Initial voltage on the VCC pin (V) 10.2 Undervoltage Lockout (UVLO) Figure 10-2 shows the relationship of the VCC pin voltage and circuit current ICC. When VCC pin voltage decreases to VCC(OFF) = 8.1 V, the control circuit stops operation by UVLO (Undervoltage Lockout) circuit, and reverts to the state before startup. Circuit current, ICC VCC（OFF） VCC（ON） VCC pin voltage Start Stop Figure 10-2 Relationship between VCC pin voltage and ICC 10.3 Bias Assist Function By the Bias Assist Function, the startup failure is prevented. When FB pin voltage is the FB/OLP Pin Oscillation Stop Threshold Voltage, VFB(OFF)= 1.23 V or less and VCC pin voltage decreases to the Startup Current Biasing Threshold Voltage, VCC(BIAS) = 9.4 V, the Bias Assist Function is activated. When the Bias Assist Function is activated, the VCC pin voltage is kept almost constant voltage, VCC(BIAS) by providing the startup current, ISTARTUP, from the startup circuit. Thus, the VCC pin voltage is kept more than VCC(OFF). Since the startup failure is prevented by the Bias Assist Function, the value of C2 connected to the VCC pin can be small. Thus, the startup time and the response time of the Overvoltage Protection (OVP) become shorter. The operation of the Bias Assist Function in startup is as follows. It is necessary to check and adjust the startup |
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