BD7F105EFJ-C
Description of Blocks – continued
3
4
5
Thermal Shutdown (TSD)
This block is the thermal shutdown circuit that prevents heat damage to the IC. When IC junction temperature rises
more than 175 °C (Typ), IC stops switching operation. After that, when IC junction temperature falls IC restarts. The
temperature hysteresis is 25 °C (Typ). The TSD function aims to protect itself. So IC junction temperature should be
designed less than Tjmax = 150 °C. For that, it should not use as over temperature protection function of application.
SW VOLTAGE DETECTION
This block detects the flyback voltage generated in the SW pin. In turn-off of the transformer, this block converts
current to flow from the FB pin into voltage by the resistance of the REF pin and the flyback voltage is detected by this
REF pin voltage.
The IC controls REF pin voltage to be equivalent to VINTREF
.
SOFT START
When IC turns to enable status that SDX/EN pin voltage rises above VEN1, the comparison voltage of the PWM
COMPARATOR block increases gradually from 0 V to VINTREF. PWM comparator voltage is constantly VINTREF after
soft start time passed.
This operation prevents from the output voltage overshooting. The soft start time is fixed to tSS in the IC.
And, SCP protection is invalid for tMASKSCP period from start-up.
6
7
PWM COMPARATOR
This block compares REF pin voltage equivalent to feedback voltage of the output voltage with soft start voltage or
reference voltage VINTREF. This comparator output decides the ON timing.
Since it does not have error amplifier and constitutes a feedback loop by the comparator, IC enables fast control to
load response during PWM operation.
ADAPTIVE ON TIME CONTROLLER
This block is ON time control block which uses original adapted ON-time control technology.
Stable load current:
Fluctuating load current:
IC operates in PWM operation by constant ON time control.
IC operates in the constant ON time control and fluctuate the switching
frequency. It results from fast response.
Light load:
The switching frequency decreases and realizes a high efficiency by PFM
operation during discontinuous mode.
When the load current fluctuates, IC operates within fSW_MAX. IC raises the primary average current by shortening the
off time. It results from increasing the secondary current and secondary output voltage is quickly stable.
Output current
Output voltage
Primary coil current
SW pin voltage
High
Frequency
Stabilize gradually
Stable operation
Switching Frequency
Stable operation at light load
Figure 3. Load Current Response Timing Chart
8
9
Maximum Frequency Limit Function (MAX FREQ)
This function limits the maximum frequency. The switching frequency is instantly high at ON width control in start-up
or load response. It may influence to EMI. For that, IC limits max frequency less than fSW_MAX
.
DRIVER
This is the block which drives Nch MOSFET for switching.
10 Nch MOSFET
This is Nch MOSFET for switching.
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