irs27951s International Rectifier Corp., irs27951s Datasheet - Page 14

• Manufacturer Part Number: irs27951s
• Description: Resonant Half-bridge Converter Control Ic
• Manufacturer: International Rectifier Corp.

Multi-function 2 Pin Oscillator
The two pin oscillator is externally programmed by a resistor RT connected between pin#2 and COM and a
capacitor CT, connected from pin#3 to COM. The RT pin provides an accurate 2V reference (±2%) with a 2mA
source capability (higher the current sourced by the pin, the higher is the oscillator frequency).
In normal operation, CT is charged by a current defined by the network connected at the RT pin. The oscillator
ramp charges and discharges between the two ramp thresholds (3 and 5V). When the ramp voltage reaches 5V,
it is internally discharged by a fixed current of 2mA. The fall time for the ramp corresponds to the dead time
between the bridge devices.
At startup, a 10uA internal current source charges the oscillator capacitor at the CT pin to V
IC is pulled out of sleep mode and the 2V reference voltage at the RT pin is enabled. The low-side device is now
also turned ON to charge the bootstrap capacitor (this sequence at startup or while exiting sleep mode ensures
that the low-side device is always switched on first to charge the bootstrap capacitor which will be ready to
supply the high-side floating driver). The low-side device remains ON till the CT pin voltage reaches the upper
ramp threshold of 5V (see Fig 2).
The programmable features for the oscillator are listed below-
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• Wide frequency range:
The high-speed oscillator allows an output frequency from 50 kHz up to 500 kHz.
• Programmable dead-time:
The oscillator timing capacitor CT also programs the dead time between LO and HO.
• Programmable soft-start time:
In resonant converter applications, the output power delivered is an inverse function of frequency
i.e. soft- start is achieved by sweeping the operating frequency from an initial high value until the
control loop takes over. Additionally, the resonant tank has a non-linear frequency dependence
that makes the converter’s power transfer capability change little when the frequency is away
from resonance and change very quickly as frequency approaches the resonant tank frequency.
An R-C series circuit (CSS + RSS) connected between the RT pin and COM programs the soft-
start time for the converter. Initially, the capacitor CSS is totally discharged, so that the series
resistor RSS is effectively in parallel to Rfmin and the resulting initial frequency is determined by
RSS and RT only (the optocoupler’s phototransistor is cut off). During this frequency sweep, the
operating frequency will decay following the exponential charge of CSS, that is, initially it will
change relatively quickly but the rate of change will get slower. The CSS capacitor charges until
its voltage reaches 2V and, consequently, the current through RSS is reduced to zero.
The soft-start sequence is activated at normal startup and back to operation from Sleep mode.
• Programmable start-up, minimum and maximum frequency:
In resonant converter applications, it is important to operate the converter in the soft-switching
region of operation. IRS2795 offers a trimmed voltage reference (±2%) at the RT pin to
accurately program the converter switching frequency range in the desired region of operation.
- The effective resistance (RSS//RT) at pin#2 during IC power up and the CT capacitor program
the start-up frequency of the converter.
- RT and CT program the minimum operating frequency.
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IRS27951S
IRS27952S