SRP Hitachi Semiconductor, SRP Datasheet - Page 15

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

Features and Theory of Current Mode Control
Features of Current Mode Control
Theory of Current Mode Control
In current mode control, a PWM pulse is generated not by comparing an error voltage with a triangular
wave voltage in the voltage mode, but by changing the current limiter level in accordance with the error
voltage (COMP terminal in this IC, that is,output of the error amplifier output) which is obtained by
constantly monitoring the current of the switching device (power MOSFET) using source resistor R
One of the features of current mode control is that the current limited operates in all cycles of PWM as
described by the above theory.
In voltage mode, only one feedback loop is made by an output voltage. In current mode, on the other hand,
two loops are used. One is an output voltage loop and the other is a loop of the switching device current
itself. The current of the switching device can be controlled switch high speed. In current mode control,
the current in the transformer winding is kept constant, resulting in high stability. An important
consequence is that the line regulation in terms of total characteristics is better than that in voltage mode.
Switch element current detection is performed every cycle, giving a high feedback response speed.
Operation with a constant transformer winding current gives a highly stable output voltage (with
excellent line regulation characteristics, in particular).
Suitable for flyback transformer use.
External synchronous operation is easily achieved. (This feature, for example, is applicable to
synchronization with a forizontal synchronizing signal of CRT monitor.)
input
AC
Figure 5 Block Diagram of Current Mode Switching Power Spply
HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
OSC
R
S
Current sense
comparator
Flip flop
RS
+
R
2R
I
S
Error amplifier
Transformar
V
COMP
+
Vref
DC
output
CS
.
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