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MAX8733EEI Fiches technique(PDF) 20 Page - Maxim Integrated Products |
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MAX8733EEI Fiches technique(HTML) 20 Page - Maxim Integrated Products |
20 / 32 page High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers 20 ______________________________________________________________________________________ Reference and Linear Regulators (REF, LDO5, and LDO3) The 2V reference (REF) is accurate to ±1% over tem- perature, making REF useful as a precision system reference. Bypass REF to GND with a 0.22µF (min) capacitor. REF can supply up to 100µA for external loads. However, if extremely accurate specifications for both the main output voltages and REF are essential, avoid loading REF. Loading REF reduces the LDO5, LDO3, OUT5, and OUT3 output voltages slightly because of the reference load-regulation error. Two internal regulators produce 5V (LDO5) and 3.3V (LDO3). LDO5 provides gate drive for the external MOSFETs and powers the PWM controller, logic, refer- ence, and other blocks within the device. The LDO5 regulator supplies a total of 100mA for internal and external loads, including MOSFET gate drive, which typically varies from 10mA to 50mA, depending on switching frequency and the external MOSFETs. LDO3 powers up when the reference (REF) is in regulation, and supplies up to 100mA for external loads. Bypass LDO5 and LDO3 with a minimum 4.7µF load; use an additional 1µF per 5mA of internal and external load. When the 5V main output voltage is above the LDO5 bootstrap-switchover threshold, an internal 1.4Ω p-chan- nel MOSFET switch connects OUT5 to LDO5 while simul- taneously shutting down the LDO5 linear regulator. Similarly, when the 3.3V main output voltage is above the LDO3 bootstrap-switchover threshold, an internal 1.5Ω p-channel MOSFET switch connects OUT3 to LDO3 while simultaneously shutting down the LDO3 linear regulator. These actions bootstrap the device, powering the internal circuitry and external loads from the output SMPS volt- ages, rather than through linear regulators from the bat- tery. Bootstrapping reduces power dissipation due to gate charge and quiescent losses by providing power from a 90%-efficient switch-mode source, rather than from a much-less-efficient linear regulator. Current-Limit Circuit (ILIM_) The current-limit circuit employs a “valley” current-sens- ing algorithm. The MAX8734 uses the on-resistance of the synchronous rectifier, while the MAX8732/MAX8733 use a discrete resistor in series with the source of the synchronous rectifier as a current-sensing element. If the magnitude of the current-sense signal at CS_ (MAX8732/MAX8733)/LX_ (MAX8734) is above the cur- rent-limit threshold, the PWM is not allowed to initiate a new cycle (Figure 7). The actual peak current is greater than the current-limit threshold by an amount equal to the inductor ripple current. Therefore, the exact current-limit characteristic and maximum load capability are a func- tion of the current-limit threshold, inductor value, and input and output voltage. For the MAX8732/MAX8733, connect CS_ to the junction of the synchronous rectifier source and a current-sense resistor to GND. With a current-limit threshold of 100mV, the accuracy is approximately ±7%. Using a lower cur- rent-sense threshold results in less accuracy. The cur- rent-sense resistor only dissipates power when the synchronous rectifier is on. For lower power dissipation, the MAX8734 uses the on- resistance of the synchronous rectifier as the current- sense element. Use the worst-case maximum value for RDS(ON) from the MOSFET data sheet, and add some margin for the rise in RDS(ON) with temperature. A good general rule is to allow 0.5% additional resistance for each °C of temperature rise. The current limit varies with the on-resistance of the synchronous rectifier. The reward for this uncertainty is robust, lossless overcur- rent sensing. When combined with the undervoltage- ON-TIME (tON) ZERO-CROSSING DETECTION ISONIC 0 40µs (MAX) INDUCTOR CURRENT Figure 6. Ultrasonic Current Waveforms ILIMIT ILOAD 0 TIME -IPEAK Figure 7. “Valley” Current-Limit Threshold Point |
Numéro de pièce similaire - MAX8733EEI |
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Description similaire - MAX8733EEI |
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