TPS25940 datasheet(26/50 Pages) TI1

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(IMON)

-6

19.23 k

5 x 52 x 10

(IMONmax)

(IMON)

-6

(LIM)

x 52 x 10

TPS25940-Q1

SLVSDJ0C – MAY 2016 – REVISED JULY 2018

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Product Folder Links: TPS25940-Q1

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From the device electrical specifications, V(OVPR) = 0.99 V and V(ENR) = 0.99 V. For design requirements, V(OV) is

16.5 V and V(UV) is 10.8 V. To solve the equation, first choose the value of R3 = 31.2 kΩ and use Equation 10 to

solve for (R1 + R2) = 488.8 kΩ. Use Equation 11 and value of (R1 + R2) to solve for R2 = 16.47 kΩ and finally R1=

472.33 kΩ.

Using the closest standard 1% resistor values gives R1 = 475 kΩ, R2 = 16.7 kΩ, and R3 = 31.2 kΩ.

The power failure threshold is detected on the falling edge of supply. This threshold voltage is 7% lower than the

rising threshold, V(UV). This is calculated using Equation 12.

V(PFAIL) = 0.93 x V(UV)

(12)

9.2.2.4 Programming Current Monitoring Resistor—RIMON

Voltage at IMON pin V(IMON) represents the voltage proportional to load current. This can be connected to an

ADC of the downstream system for health monitoring of the system. The R(IMON) need to be configured based on

the maximum input voltage range of the ADC used. R(IMON) is set using Equation 13.

(13)

For I(LIM) = 5 A, and considering the operating range of ADC from 0 V to 5 V, V(IMONmax) is 5 V and R(IMON) is

determined by:

(14)

Selecting R(IMON) value less than determined by Equation 14 ensures that ADC limits are not exceeded for

maximum value of load current.

If the IMON pin voltage is not being digitized with an ADC, R(IMON) can be selected to produce a 1V/1A voltage at

the IMON pin, using Equation 13.

Choose closest 1 % standard value: 19.1 kΩ.

If current monitoring up to I(FASTRIP) is desired, R(IMON) can be reduced by a factor of 1.6, as in Equation 5.

9.2.2.5 Setting Output Voltage Ramp Time (tdVdT)

For a successful design, the junction temperature of device must be kept below the absolute-maximum rating

during both dynamic (start-up) and steady state conditions. Dynamic power stresses often are an order of

magnitude greater than the static stresses, so it is important to determine the right start-up time and in-rush

current limit required with system capacitance to avoid thermal shutdown during start-up with and without load.

The ramp-up capacitor C(dVdT) needed is calculated considering the two possible cases Case1: Start-up Without

Load: Only Output Capacitance C(OUT) Draws Current During Start-up and Case 2: Start-Up With Load: Output

Capacitance C(OUT) and Load Draws Current During Start-Up.

9.2.2.5.1

Case1: Start-up Without Load: Only Output Capacitance C(OUT) Draws Current During Start-up

During start-up, as the output capacitor charges, the voltage difference across the internal FET decreases, and

the power dissipated decreases as well. Typical ramp-up of output voltage V(OUT) with inrush current limit of 1.2 A

and power dissipated in the device during start-up is shown in Figure 52. The average power dissipated in the

device during start-up is equal to area of triangular plot (red curve in Figure 52) averaged over tdVdT.

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