HA16116FP Datasheet, PDF (16/34 Pages) Hitachi Semiconductor – Switching Regulator for Chopper Type DC/DC Converter

HA16116FP/FPJ, HA16121FP/FPJ
6.3 Calculating Intermittent Operation Timing
Intermittent operation timing is calculated as follows.
(1) T_ON time (the time until the IC is shut off when continuous overcurrent occurs)
T_ON = C_ON/OFF × R_B × ln
3V_BE
2V_BE
×
1
1 − On duty*
1
1
= C_ON/OFF × R_B × ln1.5 × _{1 − On duty*} ≈ 0.4 × C_ON/OFF × R_B × _{1 − On duty*}
(2) T_OFF time (when the IC is off, the time until it next goes on)
T_OFF = C_ON/OFF × (R_A + R_B) × ln
V_IN − 2V_BE
V_IN − 3V_BE
Where, V_BE ≈ 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 T_ON when the pulse-by-pulse current
limiter goes into effect (resulting in a larger overload), the smaller the value T_ON becomes.
As seen in the second equation (2), T_OFF is a function of V_IN. Further, according to this setting, when V_IN is
switched on, the IC goes on only after T_OFF has elapsed.
Triangle wave
PWM output
(step-down channel)
t_ON
T
Dead band voltage
Point at which current
limiter operate
On duty = ^{t ON}
T
Where T = 1/f _OSC
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.
Figure 6.3
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