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MAX1955 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1955 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 22 page 1.6V to 5.5V Input, 0.5% Accurate, Dual 180° Out-of-Phase Step-Down Controllers ______________________________________________________________________________________ 11 Output voltage margining shifts the output voltage by ±4% from the nominal value to simplify system testing. Outputs also can be powered up and down in select- able sequences to meet core and logic supply rail requirements. DC-to-DC PWM Controller The MAX1955/MAX1956 step-down DC-to-DC convert- ers use a PWM voltage-mode control scheme. The con- troller generates the clock signal by dividing down the internal oscillator (or SYNC signal when using an external clock) so that each controller’s switching frequency equals 1/2 the oscillator frequency. An internal transcon- ductance error amplifier produces an integrated error voltage at the COMP_ pin, providing high DC accuracy. The voltage at COMP sets the duty cycle, using a PWM comparator and a ramp generator. At the rising edge of the clock, Regulator 1’s high-side N-channel MOSFET turns on and remains on until either the appropriate duty cycle or the maximum duty cycle is reached. Regulator 2 operates out of phase, so its high-side MOSFET turns on at the falling edge of the clock. During the on-time of each high-side MOSFET, the associated inductor current ramps up. During the second half of the switching cycle, the high- side MOSFET turns off and the low-side N-channel MOSFET (synchronous rectifier) turns on. The inductor releases its stored energy as its current ramps down, providing current to the load. High-Side Gate-Drive Supply (BST) The gate-drive voltage for the high-side N-channel switch is generated by a flying capacitor. This capacitor between BST and LX is alternately charged from the VDD supply and placed in parallel to the high-side MOSFET’s gate and source terminal through the high-side driver. On startup, the low-side MOSFET forces LX to ground and charges the boost capacitors to VDD through the Schottky diodes (D1 and D2 of Figure 5). On the second half cycle, the controller turns on the high-side MOSFET by closing an internal switch between BST and DH. This provides the necessary gate-to-source voltage to turn on the high-side MOSFET, an action that boosts the 5V gate-drive signal above the input voltage. Current Limit The current-limit circuit employs a “valley” current- sensing algorithm that uses the on-resistance of the low-side MOSFET as a current-sensing element. If the current-sense signal (measured from PGND_ to LX_) is above the current-limit threshold, the MAX1955/ MAX1956 do not initiate a new cycle, and COMP_ is pulled to ground. Since valley current sensing is used, the actual peak current is greater than the current-limit threshold by an amount equal to the inductor ripple current (Figure 2). The exact current-limit characteristic and maximum load capacity are a function of the low- side MOSFET’s on-resistance, the current-limit thresh- old, the inductor value, and the input voltage. This provides a robust lossless current sense that does not require current-sense resistors. An added feature is the implementation of Schottky diodes D3 and D4 (as shown in Figure 5), which reduce output short-circuit currents. Constant-Current Limit The adjustable current limit accommodates MOSFETs with a wide range of on-resistance values. The current- limit threshold is adjusted with an external resistor con- nected from ILIM_ to GND (RILIM_). The adjustment range is 75mV to 300mV, measured across the low-side MOSFET. The value of RILIM_ is calculated using the fol- lowing formula: where IVALLEY is the valley current limit and RDS(ON) is the on-resistance of the low-side MOSFET. To avoid reaching the current limit at a lower current than expected, use the maximum value for RDS(ON) at an elevated junction temperature. Refer to the MOSFET manufacturer’s data sheet for maximum values. R I A R ILIM VALLEY DS ON _( ) . = × × 015 5 µ IPEAK ILOAD IVALLEY TIME Figure 2. Inductor Current Waveform |
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