Datasheet MAX1684, MAX1685 (Maxim) - 8

Low-Noise, 14V Input, 1A, PWM Step-Down Converters
synchronization to an external clock. The MAX1684/MAX1685 can operate in five modes. Setting 0.1µF the devices to operate in the appropriate mode for the intended application (Table 1) achieves highest effi- 1 L INPUT OUTPUT ciency. 10 14V MAX CVH µH* 3 13, 14 3.3V AT 1A IN LX
PWM Control
22µF 2 C MBRS OUT The MAX1684/MAX1685 use an oscillator-triggered min- AIN 100µF 130LT3 imum/maximum on-time current-mode control scheme 16 0.1 (Figure 2). The minimum on-time is typically 220ns µF MAX1685 PGND 5 unless the regulator is in dropout. The maximum on-time ON/OFF 15 AGND SHDN is 2 / fOSC, allowing operation to 100% duty cycle. 4 12 CVL BOOT Current-mode feedback provides cycle-by-cycle current 11 limiting for superior load- and line-transient response. 1µF STBY 7 9 SYNC/PWM FB At each falling edge of the internal oscillator, the inter- nal P-channel MOSFET (main switch) turns on. This ILIM/SS CC REF allows current to ramp up through the inductor to the 10 8 6 load and stores energy in a magnetic field. The switch
MAX1684/MAX1685
0.1µF 0.1µF remains on until either the current-limit comparator 0.01 (OPTIONAL) µF trips, the maximum on-time expires, or the PWM com- parator signals that the output is in regulation. When *SUMIDA the switch turns off during the second half of each CD54-100; cycle, the inductor’s magnetic field collapses, releasing USE 22µH FOR MAX1684 the stored energy and forcing current through the out- put diode to the output filter capacitor and load. The Figure 1. Standard Application Circuit output filter capacitor stores charge when the inductor current is high and releases it when the inductor cur-
_______________Detailed Description
rent is low, smoothing the voltage across the load. The MAX1684/MAX1685 step-down, PWM DC-DC con- During normal operation, the MAX1684/MAX1685 regu- verters provide an adjustable output from 1.25V to the late the output voltage by switching at a constant fre- input voltage. They accept inputs from 2.7V to 14V and quency and modulating the power transferred to the deliver up to 1.6A. An internal MOSFET and synchro- load on each cycle using the PWM comparator. A multi- nous rectifier reduce PC board area while maintaining input comparator sums three weighted differential sig- high efficiency. Operation with up to 100% duty cycle nals (the output voltage with respect to the reference, minimizes dropout voltage. Fixed-frequency PWM oper- the main switch current sense, and the slope-compen- ation reduces interference in sensitive communications sation ramp) and changes states when a threshold is and data-acquisition applications. A SYNC input allows reached. It modulates output power by adjusting the
Table 1. Operating Modes TYPICAL MODE SYNC/PWM
STBY SHDN
FUNCTION OUTPUT CAPABILITY (A)
PWM H H H Fixed-frequency PWM 1.6 Sync PWM Clocked H H Fixed-input clock-frequency PWM 1.6 PFM at light loads (<150mA); fixed- Normal L H H 1.6 frequency PWM at heavy loads (>150mA) Low Power X L H Low-power or standby mode 160m Shutdown X X L Circuit disabled 0
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