SRP Hitachi Semiconductor, SRP Datasheet - Page 23

• Manufacturer Part Number: SRP
• Description: High Speed Current Mode PWM Control IC for Switching Power Supply
• Manufacturer: Hitachi Semiconductor

Calculation of operation parameters
1. Maximum ON duty Du max (Refer to the right figure.)
2. Oscillation frequency fosc
From the above two equations, the following two equations are
obtained.
3. Equalization to device R
4. Equation to device C
5. Operating current I
I
Example 1: Calculation when R
Example 2: Calculation for 50% of Du max and 200 kHz of fosc
The current limit is expressed as
Du max =
fosc =
R
C
IN
T
T
= I
=
= 1.78
e
Q
0.56 (1/Du max
C
+ Isink
HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
providing that I
Ciss is the input gate capacitance of the power MOSFET which is connected and V
the supply voltage of the IC.
fosc = 52kHz, Du max = 95%, I
R
However, Ciss = 1000pF, V
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.
T
T
(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
= 693 , C
1 + 1.78
R
fosc
Du max
T
CT
190
switching device to around the maximum value (1.0V Typ).
Figure 11 Calculation of Operation Parameters
{
IN
(1
0.56 + In 1 +
R
T
T
T
In 1 +
from fosc and R
Q
Du max) + Ciss
= 6360pF, I
T
1)
(
= 8.4mA Typ (Supply current when oscillation in IC stops.)
1
from Du max
1
1
(
R
+ 440
T
(e = 2.71828.base of natural logarithm)
T
190
= 10k
R
IN
440
ID
IN
T
190
max
= 12.5mA
= 18V
T
IN
440
)
V
=
= 9.7mA
and C
IN
V
R
)
THCS
}
CS
fosc
T
= 3300pF
Triangular wave
PWM maximum
ON pulse
Dumax is the ratio of
maximum ON time of
PWM to one cycle time.
In the above case,
Dumax = 95%
IN
is
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