6.3 Calculating Intermittent Operation Timing
Intermittent operation timing is calculated as follows.
(1) TON time (the time until the IC is shut off when continuous overcurrent occurs)
TON = CON/OFF × RB × ln
1 − On duty*
= CON/OFF × RB × ln1.5 × 1 − On duty* ≈ 0.4 × CON/OFF × RB × 1 − On duty*
(2) TOFF time (when the IC is off, the time until it next goes on)
TOFF = CON/OFF × (RA + RB) × ln
VIN − 2VBE
VIN − 3VBE
Where, VBE ≈ 0.7 V
Note: 1. On duty is the percent of time the IC is on during one PWM cycle when the pulse-by-pulse
current limiter is operating.
From the first equation (1) above, it is seen that the shorter the time TON when the pulse-by-pulse current
limiter goes into effect (resulting in a larger overload), the smaller the value TON becomes.
As seen in the second equation (2), TOFF is a function of VIN. Further, according to this setting, when VIN is
switched on, the IC goes on only after TOFF has elapsed.
Dead band voltage
Point at which current
On duty = tON
Where T = 1/fOSC
Note: On duty is the percent of time the IC is on during one PWM cycle when
the pulse-by-pulse current limiter is operating.
Rev.2.0, Sep.18.2003, page 16 of 33