M7085(2012) データシートの表示（PDF） - Unisonic Technologies

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M7085
(Rev.:2012)

PFM STEP-DOWN DC-DC CONTROLLER

Unisonic Technologies 

M7085

CMOS IC

„ APPLICATION INFORMATION(Cont.)

Setting over current protection threshold by external sense resistor

The VDS of a P-Channel will tend to vary significantly over temperature. This will result an equivalent variation in

current limit. To improve current limit accuracy an external sense resistor can be connected from VIN to the source of

the P-Channel, as shown in Fig. 2.

Setting start up time

The current limit circuit is active during start-up. During start-up the P-Channel will stay on until either the current

limit or the feedback comparator is tripped If the current limit comparator is tripped first then the fold back

characteristic should be taken into account. Start-up into full load may require a higher current limit set point or the

load must be applied after start-up.

One problem with selecting a higher current limit is inrush current during start-up. Increasing the capacitance(CADJ)

in parallel with RADJ results in soft-start. CADJ and RADJ create an RC time constant forcing current limit to activate at

a lower current. The output voltage will ramp more slowly when using the soft-start functionality. There are example

start-up plots for CADJ equal to 1nF and 10nF in the Typical Performance Characteristics. Lower values for CADJ will

have little to no effect on soft-start.

Inductor selection (L1)

UTC M7085 operates over a wide frequency range and can use a wide range of inductance values. The

inductance value and the resulting ripple is one of the key parameters controlling operating frequency. The second is

the ESR.

Output capacitor selection (COUT)

The output capacitor, COUT, is required to keep the output voltage ripple small and to ensure regulation loop

stability. COUT must have low impedance at the switching frequency. For most applications, a 200mF capacitor is

sufficient.

Input capacitor selection (CIN)

The input capacitor, CIN, reduces the current peaks drawn and reduces switching noise in the IC. a bypass

capacitor is required between the input source and ground. It must be located near the source pin of the external

P-Channel. The input capacitor prevents large voltage transients at the input and provides the instantaneous current

when the P-Channel turns on. The capacitance value should be selected such that the ripple voltage created by the

charge and discharge of the capacitance is less than 10% of the total ripple across the capacitor.

Catch diode selection (D1)

The important parameters for the catch diode are the peak current, the peak reverse voltage, and the average

power dissipation. For high temperature applications, diode leakage current may become significant and require a

higher reverse voltage rating to achieve acceptable performance.

UNISONIC TECHNOLOGIES CO., LTD

www.unisonic.com.tw

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