SM6702 Datasheet, PDF (5/10 Pages) Nippon Precision Circuits Inc – PFM Step-up DC/DC Converter

SM6702 series
FUNCTIONAL DESCRIPTION
Step-up Principle
The SM6702 series use a transistor to switch the input supply in a non-isolated chopper switching regulator
configuration. The figure below shows the basic step-up switching regulator circuit.
If the transistor is ON for an interval t_ON and then instantly turned OFF, the energy stored in the coil L during
the interval t_ON generates a voltage V_L across the coil. This voltage is superimposed on the input voltage such
that the peak voltage reaches V_IN + V_L which forces the diode D to conduct and release the stored energy into
the output.
During normal operation, the above switching occurs repeatedly at a frequency controlled by an oscillator
(which can change depending on the input and output load conditions), generating a square wave output which
is then smoothed by the output capacitor C. Thus the output voltage (V_OUT > V_IN) has been stepped up.
L
D
VIN
VOUT
Tr
C
GND
PFM Control
In SM6702 series, the LX transistor ON interval is fixed, and the OFF interval varies in response to the input
voltage and load changes. Thus the resulting switching cycle frequency varies, called pulse frequency modula-
tion.
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