SIP4612ASLASHB datasheet(4/10 Pages) VISHAY

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Document Number: 74481

S-80971-Rev. B, 24-Apr-08

Vishay Siliconix

SiP4612A/B

DETAILED DESCRIPTION

The SiP4612A/B limits load current by sampling the pass

transistor current and passing that through an external

resistor, RSET. The voltage across RSET, VSET, is then

compared with an internal reference voltage, VREF. In the

event that load current surpasses the set limit current, VSET

will exceed VREF causing the pass transistor gate voltage to

increase, thereby reducing the gate to source voltage of the

PMOS switch and regulating its current back down to ILIMIT.

Setting the Current Limit Level

Setting the current limit level on the SiP4612A/B requires

some care to ensure the maximum current required by the

load will not trigger the current limit circuitry. The minimum

current limit threshold should be determined by taking the

maximum current required by the load, ILOAD, and adding

25 % headroom. The SiP4612A/B has a current limit

tolerance of 25 %, which is largely a result of process

variations from part to part, and also temperature and

VIN/VOUT variances. Thus, to ensure that the actual current

limit is never below the desired current limit a 1/0.75 = 1.33

coefficient needs to be added to the calculations. Knowing

the maximum load current required, the value of RSET is

calculated as follows.

RSET = RSET coefficient/ILIMIT

where ILIMIT = (ILOAD x 1.33) x 1.25 and RSET coefficient is

3460 for a 500 mA current limit. For typical RSET coefficient

values

given

limit

current

refer

the

"Typical

Characteristics" section.

Operation at Current Limit and Thermal Shutdown

In the event that a load higher than ILIMIT is demanded of the

SiP4612A/B, the load current will stay fixed at the current

limit established by RSET. However, since the required

current is not supplied, the voltage at OUT will drop. The

increase in VIN - VOUT will cause the chip to dissipate more

heat. The power dissipation for the SiP4612A/B can be

expressed as

P = ILOAD x (VIN - VOUT)

Once this exceeds the maximum power dissipation of the

package, the die temperature will rise. When the die

temperature exceeds an over-temperature limit of 165 °C,

the SiP4612A/B will shut down until it has cooled down to

145 °C, before starting up again. As can be seen in the figure

below, the SiP4612A/B will continue to cycle on and off until

the load is reduced or the part is turned off (See Figure 2).

The maximum power dissipation in any application is

dependant

the

maximum

junction

temperature,

TJ(MAX) = 125 °C, the junction-to-ambient thermal resistance

for the TSC75-6 package,

θJ-A = 131 °C/W, and the ambient

temperature, TA, which may be formulaically expressed as:

It then follows that assuming an ambient temperature of

70 °C, the maximum power dissipation will be limited to about

419 mW.

Reverse Voltage

The SiP4612A/B is designed to control current flowing from

IN to OUT. If the voltage on OUT is raised higher than IN

current will flow from OUT to IN but the current limit function

will not be available, as can be inferred from the block

diagram in Figure 1. Thus, in applications were OUT is used

to charge IN, careful considerations must be taken to limit

current through the device and protect it from becoming

damaged.

131

125

(max)

−

Figure 2. Current Over load Condition. Load Switch turned on with 0.1

Ω load at time = 0 ms.

20 ms/div

RSET = 3.32 k

VOUT = (1 V/div)

IOUT (500 mA/div)

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