HA17384SPS Datasheet, PDF (11/29 Pages) Hitachi Semiconductor – High Speed Current Mode PWM Control IC for Switching Power Supply

HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
Operation (Description of Timing Chart)
From Power ON to Turn On
After the power is switched ON, the power supply terminal voltage (V_IN) of this IC rises by charging through bleeder
resistor R_B. At this time, when the power voltage is in the range of 2 V to 16 V*¹. The low-voltage, lock out UVL1
operates and accordingly the OUT voltage, that is, the gate voltage of the power MOS FET, is fixed at 1.3 V or a lower
value, resulting in the power MOS FET remaining in the OFF state.
When the power supply voltage reaches 16 V, UVL1 of this IC is reset and the reference voltage (Vref) generating part
turns ON. However, until Vref becomes 4.7 V, the low-voltage, lock out UVL2 operates to keep the OUT terminal
voltage low. After Vref terminal voltage becomes 4.7 V or higher, OUT terminal outputs a PWM pulse.
Note: 1. The value is for the HA17384S/H.
The value is 8.4 V for the HA17385H.
Generation of Triangular Wave and PWM Pulse
After the output of the Vref, each blocks begins to operate. The triangular wave is generated on the R_T/C_T terminal.
For PWM pulses, the triangular wave rise time is taken as the variable on-duty on-time. The triangular wave fall time is
taken as the dead-band time. The initial rise of the triangular wave starts from 0 V, and to prevent a large on-duty at
this time, the initial PWM pulse is masked and not output. PWM pulses are outputted after the second triangular wave.
The above operation is enabled by the charge energy which is charged through the bleeder resistor R_B into the capacitor
C_B of V_IN.
Stationary Operation
PWM pulses are outputted after the second wave of the triangular wave and stationary operation as the switching power
supply starts.
By switching operation from ON/OFF to OFF/ON in the switching device (power MOS FET), the transformer converts
the voltage. The power supply of IC V_IN is fed by the back-up winding of the transformer.
In the current mode of the IC, the current in the switcing device is always monitored by a source resistor R_CS. Then the
current limiter level is varied according to the error voltage (COMP terminal voltage) for PWM control. One third of
the error voltage level, which is divided by resistors “2R” and “R” in the IC, is used to sense the current (R = 25 kΩ).
Two diodes between the error output and the 2R-R circuit act only as a DC level shifter. Actually, these diodes are
connected between the 2R-R circuit and GND, and, the current sensing comparator and GND, respectively. Therefore,
these blocks operate 1.4 V higher than the GND level. Accordingly, the error of the current sensing level caused by the
switching noise on the GND voltage level is eliminated. The zener diode of 1 V symbolically indicates that the
maximum sensing voltage level of the CS terminal is 1 V.
Rev.3.00 Jun 15, 2005 page 11 of 28