STR-A6000MZ datasheet(12/26 Pages) SANKEN

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STR-A6000MZ/HZ Series

STR-A6000MZ/HZ - DS Rev.1.2

SANKEN ELECTRIC CO.,LTD.

Mar.13, 2015

9.2

Undervoltage Lockout (UVLO)

Figure 9-4 shows the relationship of VCC pin voltage

and circuit current ICC. When the VCC pin voltage

decreases to VCC(OFF) = 8.5 V, the control circuit stops

operation by the Undervoltage Lockout (UVLO) circuit,

and reverts to the state before startup.

Circuit current, ICC

ICC（ON）

VCC（OFF）

VCC（ON）

VCC pin

voltage

Start

Stop

Figure 9-4 Relationship between

VCC pin voltage and ICC

9.3

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)= 1.16 V or less and

VCC pin voltage decreases to the Startup Current

Biasing Threshold Voltage, VCC(BIAS) = 9.6 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, ICC(ST), 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

process based on actual operation in the application, so

that poor starting conditions may be avoided.

Figure 9-5 shows the VCC pin voltage behavior during

the startup period.

After the VCC pin voltage increases to VCC(ON) = 15.0

V at startup, the IC starts the operation. Then circuit

current increases and the VCC pin voltage decreases. At

the same time, the auxiliary winding voltage, VD,

increases in proportion to output voltage. These are all

balanced to produce the VCC pin voltage.

When

the

VCC

voltage

decrease

VCC(OFF) = 8.5 V in startup operation, the IC stops

switching operation and a startup failure occurs.

When the output load is light at startup, the output

voltage may become more than the target voltage due to

the delay of feedback circuit. In this case, the FB pin

voltage is decreased by the feedback control. When the

FB pin voltage decreases to VFB(OFF) or less, the IC stops

switching operation and the VCC pin voltage decreases.

When the VCC pin voltage decreases to VCC(BIAS), the

Bias Assist Function is activated and the startup failure is

prevented.

IC starts operation

VCC pin

voltage

VCC(ON)

VCC(BIAS)

VCC(OFF)

Startup failure

Startup success

Target operating

voltage

Time

Bias assist period

Increase with rising of

output voltage

Figure 9-5 VCC pin voltage during startup period

9.4

Soft Start Function

Figure 9-6 shows the behavior of VCC pin voltage and

drain current during the startup period.

VCC(ON)

VCC(OFF)

Time

VCC pin

voltage

Startup of SMPS

Normal opertion

D/ST pin

current, ID

tLIM < tOLP (min.)

Soft start period

approximately 8.75 ms (fixed)

Time

Startup of IC

tSTART

Limited by OCP operation

Figure 9-6 VCC and ID behavior during startup

The IC activates the soft start circuitry during the

startup period. Soft start time is fixed to around 8.75 ms.

during the soft start period, over current threshold is

increased step-wisely (7 steps). This function reduces the

voltage and the current stress of a power MOSFET and a

secondary side rectifier diode.

Since the Leading Edge Blanking Function (refer to

Section 9.6) is deactivated during the soft start period,

there is the case that ON time is less than the leading

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