RS2138 Datasheet, PDF (7/10 Pages) Orister Corporation – High Precision CC/CV Primary‐Side PWM Power Switch

Page No. : 7/10
Refer to the equation 3, the change of the primary winding inductance results in the change of the maximum output power
and the constant output current in CC mode. To compensate the change from variations of primary winding inductance, the
switching frequency is locked by an internal loop such that the switching frequency is
FSW = ¹
2TDEMAG
Since T_Demag is inversely proportional to the inductance, as a result, the product L_P and F_sw is constant, thus the maximum
output power and constant current in CC mode will not change as primary winding inductance changes. Up to +/‐10%
variation of the primary winding inductance can be compensated.
Frequency Jiggling for EMI improvement
The Frequency Jiggling (switching frequency modulation) is implemented in RS2138. The oscillation frequency is modulated
so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI and therefore eases the
system design.
Current Sensing and Leading Edge Blanking
Cycle‐by‐Cycle current limiting is offered in RS2138 current mode PWM control. The switch current is detected by a sense
resistor into the CS pin. An internal leading edge blanking circuit chops off the sensed voltage spike at initial internal power
MOSFET on state so that the external RC filtering on sense input is no longer needed. The PWM duty cycle is determined by
the current sense input voltage and the EA output voltage.
Gate Drive
The internal power MOSFET in RS2138 is driven by a dedicated gate driver for power switch control. Too weak the gate drive
strength results in higher conduction and switch loss of MOSFET while too strong gate drive compromises EMI.
A good tradeoff is achieved through the built‐in totem pole gate design with right output strength control.
Programmable Cable drop Compensation
In RS2138, cable drop compensation is implemented to achieve good load regulation. An offset voltage is generated at INV by
an internal current flowing into the resister divider. The current is inversely proportional to the voltage across pin COMP, as a
result, it is inversely proportional to the output load current, thus the drop due to the cable loss can be compensated. As the
load current decreases from full‐load to no‐load, the offset voltage at INV will increase. It can also be programmed by
adjusting the resistance of the divider to compensate the drop for various cable lines used.
Protection Control
Good power supply system reliability is achieved with its rich protection features including Cycle‐by‐Cycle current limiting
(OCP), VDD clamp, Power on Soft Start, and Under Voltage Lockout on VDD(UVLO).
VDD is supplied by transformer auxiliary winding output. The output of RS2138 is shut down when VDD drops below UVLO
(ON) limit and Switcher enters power on start‐up sequence thereafter.
DS‐RS2138‐03 September, 2009
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