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MAX8734A Datasheet(PDF) 18 Page - Maxim Integrated Products |
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MAX8734A Datasheet(HTML) 18 Page - Maxim Integrated Products |
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18 / 33 page  High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers 18 ______________________________________________________________________________________ These internal blocks are not powered directly from the battery. Instead, the 5V (LDO5) linear regulator steps down the battery voltage to supply both internal circuit- ry and the gate drivers. The synchronous-switch gate drivers are directly powered from LDO5, while the high- side switch gate drivers are indirectly powered from LDO5 through an external diode-capacitor boost cir- cuit. An automatic bootstrap circuit turns off the 5V lin- ear regulator and powers the device from OUT5 when OUT5 is above 4.56V. Free-Running, Constant On-Time PWM Controller with Input Feed-Forward The Quick-PWM control architecture is a pseudo-fixed- frequency, constant on-time, current-mode type with voltage feed-forward. The Quick-PWM control architec- ture relies on the output ripple voltage to provide the PWM ramp signal; thus, the output filter capacitor’s ESR acts as a current-feedback resistor. The high-side switch on-time is determined by a one-shot whose peri- od is inversely proportional to input voltage and directly proportional to output voltage. Another one-shot sets a minimum off-time (300ns typ). The on-time, one-shot triggers when the following conditions are met: the error comparator is low, the synchronous rectifier current is below the current-limit threshold, and the minimum off- time one-shot has timed out. On-Time, One-Shot (tON) Each PWM core includes a one-shot that sets the high- side switch on-time for each controller. Each fast, low- jitter, adjustable one-shot includes circuitry that varies the on-time in response to battery and output voltage. The high-side switch on-time is inversely proportional to the battery voltage as measured by the V+ input, and proportional to the output voltage. This algorithm results in a nearly constant switching frequency despite the lack of a fixed-frequency clock generator. The benefit of a constant switching frequency is the frequency can be selected to avoid noise-sensitive frequency regions: See Table 2 for approximate K-factors. The constant 0.075V is an approximation to account for the expected drop across the synchronous-rectifier switch. Switching frequency increases as a function of load current due to the increasing drop across the synchronous rectifier, which causes a faster inductor-current discharge ramp. On-times translate only roughly to switching frequen- cies. The on-times guaranteed in the Electrical Characteristics are influenced by switching delays in the external high-side power MOSFET. Also, the dead- time effect increases the effective on-time, reducing the switching frequency. It occurs only in PWM mode (SKIP = VCC) and during dynamic output voltage transitions when the inductor current reverses at light or negative load currents. With reversed inductor current, the inductor’s EMF causes LX to go high earlier than nor- mal, extending the on-time by a period equal to the DH- rising dead time. For loads above the critical conduction point, the actual switching frequency is: where VDROP1 is the sum of the parasitic voltage drops in the inductor discharge path, including synchronous rectifier, inductor, and PC board resistances; VDROP2 is the sum of the parasitic voltage drops in the charging path, including high-side switch, inductor, and PC board resistances, and tON is the on-time calculated by the MAX8732A/MAX8733A/MAX8734A. Automatic Pulse-Skipping Switchover (Idle Mode) In Idle Mode (SKIP = GND), an inherent automatic switchover to PFM takes place at light loads. This switchover is affected by a comparator that truncates the low-side switch on-time at the inductor current’s zero crossing. This mechanism causes the threshold between pulse-skipping PFM and nonskipping PWM operation to coincide with the boundary between con- f VV tV V OUT DROP ON DROP = + ++ () 1 2 t KV V V ON OUT = + () + 0 075 . SMPS SWITCHING FREQUENCY (kHz) K-FACTOR (µs) APPROXIMATE K- FACTOR ERROR (%) MAX8732A/MAX8734A (tON = VCC), 5V 200 5.0 ±10 MAX8732A/MAX8734A (tON = VCC), 3.3V 300 3.3 ±10 MAX8733A/MAX8734A (tON = GND), 5V 400 2.5 ±10 MAX8733A/MAX8734A (tON = GND), 3.3V 500 2.0 ±10 Table 2. Approximate K-Factor Errors |
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