I designed electric schematic diagram than input voltage is 40~55V ,and output voltage is 24V ,schematic as figure below:
The problem is that output voltage is unstable , and LM25037's OUTA &OUTB PIN output the maximal duty cycle is 50%, it cannot be set by RT2 resistance,
Question: Is it that LM25037's OUTA &OUTB PIN output duty cycle maximal 50%? if not is what limit this dury cycle except RT2 resistance?
Section 7.4.5 of the datasheet says the maximum duty cycle of OUTA and OUTB are inherently limited to less than 50% and is based on the value of the RT2 resistor.
Based on your schematic: For RT2=41.2k I calculate dead-time, DT=206ns. For RT1=15k I calculate Fosc=379.4kHz.
I can solve Equation 1 in the datasheet for Dmax = 92.18%. This is the duty cyle of OUT1 + OUT2. So ~46% from each OUTx.
Is this schematic implementing a buck regulator. I'm not sure the LM25037 can do this.
Regards,
Eric
Hi Jianlin,
1) Yes. OUT1 (and OUT2) maximum duty cycle is less than 50%. Section 7.4.5 of the datasheet says this.
2) Unfortunately, your design needs a (minimum) duty cycle of Vout/Vin, 24V/48V = 50%, higher considering losses. The LM25037 does not specify synchronous buck in its applications. It's meant to be used for "Full-Bridge, Half-Bridge, Push-Pull" topologies.
OUT1 and OUT2 alternate with each having the same on time. The typical Push-Pull topology is shown here.
Eric
