HA17384SPS Datasheet, PDF (14/29 Pages) Hitachi Semiconductor – High Speed Current Mode PWM Control IC for Switching Power Supply

HA17384SPS/SRP, HA17384HPS/HRP, HA17385HPS/HRP
Features and Theory of Current Mode Control
Features of Current Mode Control
• 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.)
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 MOS FET) using source resistor R_CS.
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.
AC
input
OSC
S
RS
Flip flop
R
Transformar
DC
output
Current sense
comparator
I_S
2R
V_COMP
Vref
R
Error amplifier
Figure 5 Block Diagram of Current Mode Switching Power Spply
Rev.3.00 Jun 15, 2005 page 14 of 28