M7085-SM1-R datasheet(6/8 Pages) UTC

6 / 8 page

M7085

CMOS IC

UNISONICTECHNOLOGIESCO.,LTD

6 of 8

www.unisonic.com.tw

QW-R502-092,A

APPLICATION INFORMATION

Setting the output voltage

Select an output voltage between 1.24V and VIN by connecting FB to a resistive voltage-divider between VOUT

and GND (see the Typical Operating Circuit). Choose R2 for a reasonable bias current in the resistive divider. A wide

range of resistor values is acceptable.

R1,R2 is given by:

VOUT = 1.240* ( R1 + R2 ) / R2

Setting over current protection threshold by the RDS(ON) of the P-Channel

The UTC M7085 has a cycle-by-cycle current limit. Current limit is sensed across the VDS of the P-Channel or

across an additional sense resistor. When current limit is activated, the UTC M7085 turns off the external P-Channel

for a period of 9µs(typical). The current limit is adjusted by an external resistor, RADJ.

The current limit circuit is composed of the ISENSE comparator and the one-shot pulse generator. The positive input

of the ISENSE comparator is the ADJ pin. An internal 5.5µA current sink creates a voltage across the external RADJ

resistor. This voltage is compared to the voltage across the P-Channel or sense resistor. The ADJ voltage can be

calculated as follows:

VADJ = VIN − (RADJ * 3.0µA)

Where 3.0µA is the minimum ICL(ADJ) value.

The negative input of the ISENSE comparator is the ISENSE pin that should be connected to the drain of the external

P-Channel. The inductor current is determined by sensing the VDS. It can be calculated as follows.

VISENSE = VIN − (RDS(ON) * IIND_PEAK) = VIN − VDS

ISENSE

COMP

5.5µA

ADJ

CADJ

RADJ

ISENSE

VDS

VIN

FIGURE 1. Current Sensing by VDS

The current limit is activated when the voltage at the ADJ pin exceeds the voltage at the ISENSE pin. The ISENSE

comparator triggers the 9µs one shot pulse generator forcing the driver to turn the P-Channel off. The driver turns

the P-Channel back on after 9µs. If the current has not reduced below the set threshold, the cycle will repeat

continuously.

A filter capacitor, CADJ, should be placed as shown in Figure 1. CADJ filters unwanted noise so that the ISENSE

comparator will not be accidentally triggered. A value of 100pF to 1nF is recommended in most applications. Higher

values can be used to create a soft-start function. The current limit comparator has approximately 100ns of blanking

time. This ensures that the P-Channel is fully on when the current is sensed. However, under extreme conditions

such as cold temperature, some P-Channels may not fully turn on within the blanking time. In this case, the current

limit threshold must be increased. If the current limit function is used, the on time must be greater than 100ns. Under

low duty cycle operation, the maximum operating frequency will be limited by this minimum on time.

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