Datasheet MAX1951 (Analog Devices) - 7

1MHz, All-Ceramic, 2.6V to 5.5V Input, 2A PWM Step-Down DC-to-DC Regulators MAX1951/MAX1952 Pin Description Controller Block Function
The MAX1951/MAX1952 step-down converters use a PWM current-mode control scheme. An open-loop com-
PIN NAME FUNCTION
parator compares the integrated voltage-feedback signal Supply Voltage. Bypass V 1 V CC with 0.1µF CC against the sum of the amplified current-sense signal and capacitor to ground and 10Ω resistor to IN. the slope compensation ramp. At each rising edge of the Reference Bypass. Bypass REF with 0.1µF internal clock, the internal high-side MOSFET turns on 2 REF capacitor to ground. until the PWM comparator trips. During this on-time, cur- rent ramps up through the inductor, sourcing current to 3 GND Ground the output and storing energy in the inductor. The current- Feedback Input. Connect FB to the output to mode feedback system regulates the peak inductor cur- regulate using the internal feedback resistor rent as a function of the output voltage error signal. Since string (MAX1952). Connect an external resistor- the average inductor current is nearly the same as the 4 FB divider from the output to FB and GND to set peak inductor current (<30% ripple current), the circuit the output to a voltage between 0.8V and VIN acts as a switch-mode transconductance amplifier. To (MAX1951). preserve inner-loop stability and eliminate inductor stair- Regulator Compensation. Connect series RC casing, a slope-compensation ramp is summed into the network from COMP to GND. Pull COMP below main PWM comparator. During the second half of the 5 COMP 0.17V to shut down the regulator. COMP = cycle, the internal high-side p-channel MOSFET turns off, GND when V and the internal low-side n-channel MOSFET turns on. IN is less than 2.25V (see the Compensation and Shutdown Mode section) The inductor releases the stored energy as its current ramps down while still providing current to the output. The Power Ground. Internally connected to GND. output capacitor stores charge when the inductor current 6 PGND Keep power ground and signal ground planes exceeds the load current, and discharges when the separate. inductor current is lower, smoothing the voltage across the load. Under overload conditions, when the inductor Inductor Connection. Connect an inductor 7 LX current exceeds the current limit (see the Current Limit between LX and the regulator output. section), the high-side MOSFET does not turn on at the Power-Supply Voltage. Input voltage range rising edge of the clock and the low-side MOSFET from 2.6V to 5.5V. Bypass IN with a 10µF (min) remains on to let the inductor current ramp down. 8 IN ceramic capacitor to GND and a 10Ω resistor
Current Sense
to VCC. An internal current-sense amplifier produces a current signal proportional to the voltage generated by the
Detailed Description
high-side MOSFET on-resistance and the inductor cur- rent (RDS(ON) x ILX). The amplified current-sense signal The MAX1951/MAX1952 high-efficiency switching regula- and the internal slope compensation signal are tors are small, simple, DC-to-DC step-down converters summed together into the comparator’s inverting input. capable of delivering up to 2A of output current. The The PWM comparator turns off the internal high-side devices operate in pulse-width modulation (PWM) at a MOSFET when this sum exceeds the output from the fixed frequency of 1MHz from a 2.6V to 5.5V input voltage voltage-error amplifier. and provide an output voltage from 0.8V to VIN, making the MAX1951/MAX1952 ideal for on-board postregula-
Current Limit
tion applications. The high switching frequency allows The internal high-side MOSFET has a current limit of 3.1A for the use of smaller external components, and internal (typ). If the current flowing out of LX exceeds this limit, synchronous rectifiers improve efficiency and eliminate the high-side MOSFET turns off and the synchronous the typical Schottky free-wheeling diode. Using the on- rectifier turns on. This lowers the duty cycle and causes resistance of the internal high-side MOSFET to sense the output voltage to droop until the current limit is no switching currents eliminates current-sense resistors, longer exceeded. A synchronous rectifier current limit of further improving efficiency and cost. The MAX1951 -0.6A (typ) protects the device from current flowing into total output error over load, line, and temperature (0°C LX. If the negative current limit is exceeded, the synchro- to +85°C) is less than 1%. nous rectifier turns off, forcing the inductor current to flow
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