SP7656EN2-LSLASHTR datasheet(5/10 Pages) EXAR

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11/07/08 SP7656 PowerBlox



Overcurrent Protection

Figure 2

– Overcurrent Protection Circuit

The overcurrent protection circuit functions by

monitoring the voltage across the internal P-

Channel FET. When this voltage exceeds

0.3V (nominal), the overcurrent comparator

triggers and the controller enters hiccup mode.

Since the FET has nominal Rds(on) of 0.06Ω,

assuming no temperature rise, the overcurrent

will trigger at Iocp = 0.3V/0.06Ω=5A. To

program a lower overcurrent use a resistor Rs

between ISET and LX pins. Calculate Rs from:





Iocp











Where:

- 1.15 is a multiplier to calculate peak current

- Iocp is the desired overcurrent protection,

- 0.06Ω is nominal FET Rsd(on) at 25°C

- Kt is a multiplier that calculates increase in

Rds(on) for a given temperature

Kt is 1.0 and 1.4 for the FET operating at 25°C

and 150°C respectively. This accounts for

40% increase in FET Rds(on) as temperature

is increased from 25°C to 150°C.

Example: Calculate Rs for Iocp of 3.5A. FET

operating temperature is 88°C.















Rs=340Ω

Using the above equation there is good

agreement between calculated and test results

for Rs up to 3k. For Rs larger than 3k test

results are lower than those predicted by the

Rs equation due to circuit parasitics. Therefore

the maximum value of Rs should be limited to

3kΩ.

Using the ON/OFF Function via VFB

The feedback pin serves a dual role of On/Off

control. The MOSFET driver is disabled when

a voltage greater than 1V is applied at FB pin.

Maximum voltage rating of this pin is 5.5V.

The controlling signal should be applied

through a small signal diode to FB pin. Please

note that an optional 10k bleeding resistor across

the

output

helps

keep

the

output

capacitor

discharged under no load condition.

Programming the Output Voltage

To program the output voltage, calculate R2

using the following equation:

















200

Vout

0.6 is used as it is the reference voltage of the

SP7656. 200k is a fixed-value and is the top

resistor in the output set point resistor pair. In

addition to being part of the voltage divider, it

is part of the compensation network. R1 of the

output voltage divider should always be

200kΩ.

Soft Start

Soft Start is preset internally to 5ms (nominal).

Internal Soft Start eliminates the need for the

external capacitor CSS that is commonly used

to program this function.

Input Capacitance Selection

Select the input capacitor for Voltage,

Capacitance, ripple current, ESR and ESL.

Voltage rating is nominally selected to be

approximately twice the input voltage. The

RMS value of input capacitor current,

assuming a low inductor ripple current (IRIP),

can be calculated from:

)

(

Iout

Icin





In general total input voltage ripple should be

maintained below 1.5% of VIN (not to exceed

180mV).

Input voltage ripple has three components:

ESR and ESL cause a step voltage drop upon

turn on of the MOSFET. During the on time

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