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MAX5003EEE Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX5003EEE Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 16 page VCC regulator has a lockout line that shorts the N-chan- nel MOSFET driver output to ground if the VCC LDO is not regulating. VCC feeds all circuits except the VCC lockout logic, the undervoltage lockout, and the power regulators. The preferred method for powering the MAX5003 is to start with the high-voltage power source (at V+ or ES, depending on the application), then use a bootstrap source from the same converter with an output voltage higher than the VDD regulator turn-off voltage (10.75V) to power VDD. This will disable the power consumption of the VDD LDO. It is also possible to power the MAX5003 with no bootstrap source from ES or V+, but do not exceed the maximum allowable power dissipa- tion. The current consumption of the part is mostly a function of the operating frequency and the type of external power switch used—in particular, the total charge to be supplied to the gate. A reference output of 3V nominal is externally available at the REF pin, with a current sourcing capability of 1mA. A lockout circuit shuts off the oscillator and the output driver if REF falls 200mV below its set value. Minimize loading at REF, since the REF voltage is the source for the FB voltage, which is the regulator set point when the error amplifier is used. Any changes in VREF will be proportionally reflected in the regulated output voltage of the converter. Undervoltage Lockout, Feed Forward, and Shutdown The undervoltage lockout feature disables the controller when the voltage at INDIV is below 1.2V (120mV hys- teresis). When INDIV rises higher than 1.2V plus the hysteresis (typically 1.32V), it allows the controller to start. An external resistive divider connected between the power line and AGND generates the INDIV signal. INDIV is also used as the signal for the fast input volt- age feed-forward circuit. Always connect INDIV to a voltage divider. It is not a “don’t care” condition; the signal is used to set the fast feed-forward circuit (see the Oscillator and Ramp Generator section). Choose R2 (Figure 2) between 25k Ω to 500kΩ and cal- culate R1 to satisfy the following equation: where VSUL = system undervoltage lockout and VINDIVLO = INDIV undervoltage lockout. The undervoltage lockout function allows the use of the INDIV pin as a shutdown pin with an external switch to ground. The shutdown circuit must not affect the resis- tive divider during normal operation. Current-Sense Comparator The current-sense (CS) comparator and its associated logic limit the current through the power switch. Current is sensed at CS as a voltage across a sense resistor between the external MOSFET source and PGND. Connect CS to the external MOSFET source through a 100 Ω resistor or RC lowpass filter (Figures 2 and 3). See CS Resistor in the Component Selection section. A blanking circuit shunts CS to ground when the power MOSFET switch is turned off, and keeps it there for 70ns after turn-on. This avoids false trips caused by the switching transients. The blanking circuit also resets the RC filter, if used. When VCS > 100mV, the power MOSFET is switched off. The propagation delay from the time the switch current reaches the trip level to the driver turn-off time is 240ns. If the current limit is not used, the CS pin must be connected to PGND. Error Amplifier The internal error amplifier is one of the building blocks that gives the MAX5003 its flexibility. Its noninverting input is biased at 1.5V, derived from the internal 3V ref- erence. The inverting input is brought outside (FB pin) and is the regulation feedback connection point. If the error amplifier is not used, connect this pin to ground. The output is available for the frequency compensation network and for connection to the input of the PWM comparator (CON). Unity-gain frequency is 1.2MHz, open-circuit gain is 80dB, and the amplifier is unity- gain stable. To eliminate long overload recovery times, there are clamps limiting the output excursions close to the range limits of the PWM ramp. The voltage at the noninverting input of the error amplifier is the regulator set point, but is not accessible. Set-point voltage can be measured, if needed, by con- necting COMP and FB and measuring that node with respect to ground. The error amplifier is powered from the VCC rail. PWM Comparator The pulse-width modulator (PWM) comparator stage transforms the error signal into a duty cycle by comparing the error signal with a linear ramp. The ramp levels are 0.5V min and 2.5V max. The comparator has a typical hysteresis of 5.6mV and a propagation delay of 100ns. The output of the comparator controls the external FET. Soft-Start The soft-start feature allows converters built using the MAX5003 to apply power to the load in a controllable soft ramp, thus reducing start-up surges and stresses. RR V V SUL INDIVLO 12 1 =− High-Voltage PWM Power-Supply Controller _______________________________________________________________________________________ 9 |
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