HA17384SPS Datasheet, PDF (22/29 Pages) Hitachi Semiconductor – High Speed Current Mode PWM Control IC for Switching Power Supply

HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
•^{Calculation of operation parameters}
1. Maximum ON duty Du max (Refer to the right figure.)
Du max =
1
( ) 1 + 1.78 × In 1 + 190Ω
R_T − 440Ω
Triangular wave
2. Oscillation frequency fosc
{ ( )} fosc =
C_T × R_T ×
1
190Ω
0.56 + In 1 + _{RT − 440Ω}
From the above two equations, the following two equations are
obtained.
3. Equalization to device R_T from Du max
R_T =
190Ω
+ 440Ω
e ^{0.56 (1/Du max − 1)}− 1
(e = 2.71828.base of natural logarithm)
PWM maximum
ON pulse
Dumax is the ratio of
maximum ON time of
PWM to one cycle time.
4. Equation to device C_T from fosc and R_T
C_T = 1.78 × ^{Du max}
fosc × R_T
In the above case,
Dumax = 95%
5. Operating current I_IN
I_IN = I_Q + Isink_CT × (1 − Du max) + Ciss × V_IN × fosc
providing that I_Q = 8.4mA Typ (Supply current when oscillation in IC stops.)
Ciss is the input gate capacitance of the power MOSFET which is connected and V_IN is
the supply voltage of the IC.
Example 1: Calculation when R_T = 10kΩ and C_T = 3300pF
fosc = 52kHz, Du max = 95%, I_IN = 9.7mA
Example 2: Calculation for 50% of Du max and 200 kHz of fosc
R_T = 693Ω, C_T = 6360pF, I_IN = 12.5mA
However, Ciss = 1000pF, V_IN = 18V
Note that the actual value may differ from the calculated one because of the internal
delay in operation and input characteristics of the POWER MOS FET. Check the
value when mounting.
Additionally a small Dumax leads to a large supply current, even if the frequency is
not changed, and start up may become difficult. In such a case, the following
measure is recommended.
(1) For an AC/DC converter, a small bleeder resistance is required.
(2) The large capacitance between Vref and GND is required.
(3) Use a large Dumax with a triangular wave and raise the current limit of the
switching device to around the maximum value (1.0V Typ).
The current limit is expressed as
IDmax = ^VTHCS
R_CS
Figure 11 Calculation of Operation Parameters
Rev.3.00 Jun 15, 2005 page 22 of 28