The high-power, synchronous boost converter MAX1703 has two operating modes: low-power PFM for low output currents (to 250mA), and full-power low-noise PWM for currents up to 1.5A. (PFM stands for pulsed-frequency modulation; PWM stands for pulse-width modulation.)
Normally, the IC's operating mode is set by digital control of its CLK/SEL input (high for high-power mode; low for low-power mode). For systems without this control capability, the Figure 1 circuit provides a simple and inexpensive method for switching automatically between the low- and high-power modes of operation. As a result, it maintains relatively high efficiency for all load currents (Figure 2).
Figure 1. Using a high-side current monitor (U2) to generate a proportional feedback voltage, this synchronous boost converter (U1) shifts automatically between its high-power and low-power modes of operation.
Figure 2. The Figure 1 circuit's conversion efficiency (C) remains high for a wide range of load current.
U2 is a high-side current-sense monitor whose OUT terminal produces a voltage proportional to the converter's load current (sensed by the 80mΩ resistor RSENSE). Applying this voltage to U1's POKIN terminal completes a feedback loop for controlling its operating mode. POKIN is the inverting input of the "power OK" comparator. Its open-drain output sinks current whenever the POKIN voltage falls below the comparator's reference voltage (1.25V).
Because U1's low-power and high-power efficiencies converge for an output current near 150mA, you should choose component values that cause the mode switchover to occur near 150mA. First, calculate the required sense resistor: RSENSE = VREF/(GAIN × ISWITCH), where VREF is U1's internal reference voltage (1.25V), GAIN is U2's internal gain level (user-selectable as 20V/V, 50V/V, or 100V/V), and ISWITCH is the load current at which mode switchover occurs.
Because U1 can deliver high output currents, GAIN is set at 100V/V to minimize the sense-resistor value and its IR loss. To switch modes at 150mA, RSENSE is calculated at 80mΩ. Measurements indicate that switchovers occur at 158.6mA for increasing currents and at 157.0mA for decreasing currents. The POK comparator's 1% hysteresis prevents "chattering" between modes when the load current is passing slowly through the switchover threshold.
U2 is powered directly from the input to improve the circuit's light-load efficiency. U1 can operate with input voltages below 1V, but U2 requires a minimum VCC of 3V. To allow operation over U1's full input range (with a slight penalty in light-load efficiency), simply power U2 from U1's output voltage.
A similar version of this article appeared in the February 18, 2002 issue of EE Times magazine.
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