Fujitsu MB39A105 manual Power, Timer-latch short-circuit protection circuit SCP, Operating circuit

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MB39A105

2. Power Control Function

A switch in series with a resistor connected with the RT terminal (pin 7) allows you to turn on or turn off the power.

On/off setting conditions of power supply

CTL

Power

 

 

L

OFF (standby)

 

 

H

ON (operating)

 

 

RT

ON/OFF CTL

(L : OFF, H : ON)

3.Protective Functions

(1)Timer-latch short-circuit protection circuit (SCP)

Short-circuit detection comparator detects the error amplifier output voltage level. If the load conditions for the DC/DC converter are stable, the short-circuit protection comparator is kept in equilibrium condition because the error amplifier is free from output variation. At this time the CSCP terminal (pin 2) is held at the soft-start end voltage (about 0.8 V) . If the DC/DC converter output voltage falls and error amplifier output is over 0.9 V, the timer circuits are actuated to start charging the external capacitor CSCP.

When the capacitor voltage reaches about 1.0 V, the latch is set and the circuit is turned off the external FET and sets the dead time to 100 %. At this time, latch input is closed and the CSCP terminal is held at the “L” level. To reset the actuated protection circuit, turn off and on the power supply again and set VCC terminal voltage (pin 3) to 1.1 V (Min) or less. (See ■SETTING TIME CONSTANT FOR TIMER-LATCH SHORT-CIRCUIT PRO-

TECTION CIRCUIT.)

(2) Under voltage lockout protection circuit (UVLO)

The transient state or a momentary decrease in supply voltage, which occurs when the power supply is turned on, may cause the IC to malfunction, resulting in breakdown or degradation of the system. To prevent such malfunctions, under voltage lockout protection circuit detects a decrease in internal reference voltage with respect to the power supply voltage, turns off the output FET, and sets the dead time to 100% while holding the CSCP terminal (pin 2) at the “L” level.

The circuit restores the output transistor to normal when the supply voltage reaches the threshold voltage of the undervoltage lockout protection circuit.

(3) Short-circuit protection detection output function

Connecting the Pch MOS FET to SCPOD terminal (pin 4) turns off the Pch MOS FET when the short-circuit protection is detected or under voltage lockout protection circuit operate. This allows you to prevent the short- circuit between the input and output when the short-circuit protection is detected, thus preventing the input voltage from occurring in the output region in the standby state.

(4) Protection circuit operating function table

This table refers to output condition when protection circuit is operating.

Operating circuit

SCPOD

OUT

 

 

 

Short-circuit protection circuit

High-Z

L

 

 

 

Under voltage lockout protection circuit

High-Z

L

 

 

 

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Contents Package FeaturesDescription PIN Assignment PIN Description Pin No Symbol DescriptionsBlock Diagram Recommended Operating Conditions Parameter Symbol Condition Rating Unit Min MaxParameter Symbol Condition Value Unit Min Typ Absolute Maximum RatingsPin Conditions Value Unit Min Typ Max Electrical CharacteristicsParameter Error Typical CharacteristicsTriangularwave oscillation Power Dissipation vs. Ambient Temperature PWM comparator block PWM Comp Error amplifier block Error Amp1, Error Amp2Functions Triangular-wave oscillator block OSCShort-circuit protection detection output function PowerTimer-latch short-circuit protection circuit SCP Under voltage lockout protection circuit UvloSetting the Triangular Oscillation Frequency Setting the Output VoltageSetting the SOFT-START Times Timer-latch short-circuit protection circuit MB39A105 Equivalent Circuit Application Parts No Parts ListSelection of Components 2I OV O 2LV O Peak-to-peak value ESR Reference Data Part number Package Remarks Usage PrecautionOrdering Information Do not apply negative voltagesIndex Package DimensionFujitsu Limited
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