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MAX17598 Datasheet(PDF) 18 Page - Maxim Integrated Products |
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MAX17598 Datasheet(HTML) 18 Page - Maxim Integrated Products |
18 / 25 page ���������������������������������������������������������������� Maxim Integrated Products 18 MAX17598/MAX17599 Low IQ, Wide-Input Range, Active Clamp Current-Mode PWM Controllers n-Channel MOSFET Gate Driver The NDRV output drives an external n-channel MOSFET. NDRV can source/sink in excess of 900mA /1500mA peak current. Therefore, select a MOSFET that yields acceptable conduction and switching losses. p-Channel MOSFET Gate Driver The AUXDRV output drives an external p-channel MOSFET with the aid of a level shifter, as shown in the Typical Application Circuits. AUXDRV can source/sink in excess of 300mA/600mA peak current. Therefore, select a MOSFET that yields acceptable conduction and switch- ing losses. The external PMOSFET used must be able to withstand the maximum clamp voltage. Dead Time Dead time between the main and AUX output edges allow ZVS to occur, minimizing switching losses and improving efficiency. The dead time (tDT) is applied to both leading and trailing edges of the main and AUX outputs as shown in Figure 7. Connect a resistor between DT and GND to set tDT to any value between 25ns and 250ns: DT DT 10k R (t ) 25ns W = × Oscillator/Switching Frequency The ICs’ switching frequency is programmable between 100kHz and 1MHz with a resistor RRT connected between RT and GND. Use the following formula to determine the appropriate value of RRT needed to generate the desired output switching frequency (fSW): 10 RT SW 1 10 R f × = where fSW is the desired switching frequency. Peak-Current-Limit The current-sense resistor (RCS), connected between the source of the n-channel MOSFET and PGND, sets the current limit. The source end of the current-sense resis- tor connects to the CS pin of the MAX17598/MAX17599. The signal thus obtained is used by the devices, both for current-mode control and peak-current limiting purposes. The current-limit comparator has a voltage trip level (VCS-PEAK) of 300mV, and is independent of slope compensation applied to stabilize the converter. The following equation is used to calculate the value of RCS: CS PRI_PEAK 300mV R 1.2 I = × where IPRI_PEAK is the peak current in the primary side of the transformer, which also flows through the main n-channel MOSFET. When the voltage produced by this current (through the current-sense resistor) exceeds the current-limit comparator threshold, the MOSFET driver (NDRV) terminates the current on-cycle within 40ns (typ). The devices implement 70ns of internal leading-edge blanking to ignore leading-edge current spikes encoun- tered in practice due to parasitics. Use a small RC network for additional filtering of the leading-edge spike on the sense waveform when needed. Set the corner frequency of the RC filter network at 5 to 10 times the switching frequency. For a given peak-current-limit setting, the runaway cur- rent limit is typically 20% higher. The peak current-limit- triggered hiccup operation is disabled until the end of soft-start, while the runaway current-limit-triggered hiccup operation is always enabled. Negative Peak Current Limit The MAX17598/MAX17599 protect against excessive negative currents through the clamp switch, primary of the transformer and the clamp capacitor under dynamic operating conditions where the converter is not in steady state. The devices limit negative current by monitoring the voltage across RCS, while the AUXDRV output is low and the p-Channel FET is on. The typical negative-cur- rent-limit threshold is set at -100mV (1/3 of the positive- peak-current-limit threshold). Figure 7. Dead Time Between AUXDRV and NDRV AUXDRV NDRV DEAD TIME, tDT |
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