JAJSOS4K December 2008 – June 2022 LM25088 , LM25088-Q1
PRODUCTION DATA
With the inductor and sense resistor value selected, the value of the ramp capacitor (CRAMP) necessary for the emulation ramp circuit is given by:
where
For the current design example, the ramp capacitor is calculated as:
The next lowest standard value 270 pF was selected for CRAMP. An NPO capacitor with 5% or better tolerance is recommended. Note that selecting a capacitor value lower than the calculated value increases the slope compensation. Furthermore, selecting a ramp capacitor substantially lower or higher than the calculated value also results in incorrect PWM operation.
For VOUT > 5 V, internal slope compensation provided by the LM25088 may not be adequate for certain operating conditions especially at low input voltages. A pullup resistor may be added from the VCC to RAMP pin to increase the slope compensation. Optimal slope compensation current can be calculated from:
and RRAMP is given by