34652 datasheet(19/26 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34652

TYPICAL APPLICATIONS

timer initiates. The UV and OV detection circuits are

equipped with a 1.0 ms filter to filter out momentary input

supply dips. Filter capacitors between the UV and VIN pins

and between the OV and VIN pins could also be added to

adjust the UV/OV filter time and prevent more transients from

affecting the device’s operation, especially if the input supply

has a lot of noise.

Guidelines for Choosing Resistor Divider Values

The total current flowing in the resistors is equal to the

supply voltage divided by the total series resistance. The

supply voltage can reach up to 80 V and the device will still

be in normal operation, the resistors connected and drawing

current. So the resistor values should be chosen high enough

to allow for a reasonable current to pass through them and

not dissipate a lot of power or cause input noise that would

trip the UV/OV detection circuit.

Another consideration is whether or not the values of the

resistors are readily available. The tolerance of the resistors

should be 1% or better to get an accurate reading.

Note Accuracy requirements are application dependent.

To demonstrate the importance of the accuracy of the

resistors, let’s look at a system with an operating range of

40 V for UV falling to 75 V for OV rising as an example. This

operating range will be scaled down for the device’s internal

circuitry to operate the UV/OV detection circuits. The scale

factor is 31.6 for UV and 57.1 for the OV. Taking overvoltage

as an example, this means that every 5.0 mV change on the

OV pin represents a 0.29 V change for the OV trip point on

the supply. Which says that an error of 5.0 mV due to the

resistors not being accurate will result in an error of 0.29 V for

the trip point, and depending on how close we are operating

to the OV rising threshold the device might detect an OV

condition and turn off the Power MOSFET prematurely. The

same argument applies to the UV pin.

Example of Calculations for Resistor Values

The following equations are examples of calculating

resistor values using the same operating range as in the

previous paragraph:

R3 = 1.3 * R1 * VUV(RISING) / (VOV(RISING) (VUV(RISING) - 1.3))

R2 = R3 (VOV(RISING) / VUV(RISING) - 1)

Where VOV(RISING) = 75 V and VUV(RISING) = 41 V

Note Some iteration may be required to get the right

values and also standard resistor values. The recommended

maximum value of the series resistance between the UV/OV

pins and VPWR pin is 500 k

Ω.

Here we have two equations and three unknowns. If we

select a value for R1 of 487 kΩ, then from the first equation:

R3 = 8.72 kΩ

and the closest 1% standard resistor value is 8.66 k

Ω.

Now, from the second equations we can solve for R2:

R2 = 7.18 kΩ

and the closest 1% standard resistor value is 7.15 k

Ω.

If the three-resistor divider, which is the recommended

approach, could not produce acceptable resistor values, the

user can consider two separate resistor dividers, one divider

for each pin from VPWR to VIN. An advantage of the two-

resistor dividers approach is that the user can set the trip

points of the UV and OV thresholds independently.

TIMER

The TIMER pin on the 34652 gives the user control over

the time base used to generate the timing sequences at start-

up. The same timer controls the retry delay when the device

experiences any fault. The TIMER pin can be left

unconnected for a default timer value of 200 ms or the user

can connect an external resistor (RTIMER) between the

TIMER and VIN pins, as shown in Figure 19, page 18, to set

the timer value externally.

After the device passes the UVLO threshold and

continuously after that, the 34652 checks the TIMER pin for

any external components to determine the value of the timer.

During start-up and if any fault occurred, this timer value is

used when initiating a start-up sequence.

Choosing the External Resistor RTIMER Value

The user can change the value of the Start-Up Delay

Timer (tTIMER) by adding a resistor (RTIMER) between the

TIMER and VIN pins, as shown in Figure 19, page 18. The

timer value ranges between 100 ms and 1000 ms, with a

default value of 200 ms. Table 6 lists examples of RTIMER for

different values of the tTIMER and Figure 20, page 20, shows

a plot of RTIMER versus tTIMER. It is recommended that the

closest 1% standard resistor value to the actual value be

chosen.

Note Accuracy requirements are application dependent.

To calculate the value of the RTIMER resistor we use the

following equations:

tTIMER(ms) = 20(ms) + 2.0 * [RTIMER(kΩ) + 1.0 kΩ]

RTIMER(kΩ) = [tTIMER(ms) - 20(ms)] / 2.0 - 1.0 kΩ

Table 6. RTIMER Values for Some Desired tTIMER Values

tTIMER(ms)

RTIMER(kΩ)

tTIMER(ms)

RTIMER(kΩ)

100

600

289

150

650

314

200

700

339

250

114

750

364

300

139

800

389

350

164

850

414

400

189

900

439

450

214

950

464

500

239

1000

489

500

264

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