OUT

INRUSH

L

dV

I

C

dt

=

´

VIN

VON

VOUT

VIN

VON

VOUT

21

TPS22810

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VIN = 12 V

CIN = 1 µF

CL = 22 µF

Figure 26. tF at VIN = 12 V, RQOD = 500 Ω

VIN = 12 V

CIN = 1 µF

CL = 22 µF

Figure 27. tF at VIN = 12 V, QOD = VOUT

10.5.2.2 VIN to VOUT Voltage Drop

The VIN to VOUT voltage drop in the device is determined by the RON of the device and the load current. The

RON of the device depends upon the VIN conditions of the device. Refer to the RON specification of the device in

the Electrical Characteristics table of this datasheet. Once the RON of the device is determined based upon the

VIN conditions, use Equation 8 to calculate the VIN to VOUT voltage drop.

∆V = ILOAD × RON

where

•

ΔV is the voltage drop from VIN to VOUT

•

ILOAD is the load current

•

RON is the On-resistance of the device for a specific VIN

(8)

An appropriate ILOAD must be chosen such that the IMAX specification of the device is not violated.

10.5.2.3 Inrush Current

To determine how much inrush current is caused by the CL capacitor, use Equation 9.

where

•

IINRUSH is the amount of inrush caused by CL

•

CL is the capacitance on VOUT

•

dt is the Output Voltage rise time during the ramp up of VOUT when the device is enabled

•

dVOUT is the change in VOUT during the ramp up of VOUT when the device is enabled

(9)

The appropriate rise time can be calculated using the design requirements and the inrush current equation. As

we calculate the rise time (measured from 10% to 90% of VOUT), we account for this in our dVOUT parameter

(80% of VOUT = 9.6 V) as shown in Equation 10 and Equation 11.

400 mA = 22 µF × 9.6 V/dt

(10)

dt = 528 µs

(11)

To ensure an inrush current of less than 400 mA, choose a CT value that yields a rise time of more than 528 μs.

Consulting Table 2 at VIN = 12 V, CT = 4700 pF provides a typical rise time of 957 μs. Using this rise time and

voltage into Equation 9, yields Equation 12 and Equation 13.

IInrush = 22 µF × 9.6 V/ 957 µs

(12)

Inrush = 220 mA

(13)