SP7656 EXAR [Exar Corporation], SP7656 Datasheet - Page 5

• Manufacturer Part Number: SP7656
• Description: 3A, 29V Non-Synchronous Buck Converter
• Manufacturer: EXAR [Exar Corporation]

Overcurrent Protection
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:
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
11/07/08 SP7656 PowerBlox
Rs
Figure 2 – Overcurrent Protection Circuit
Rs

0

3 .
0

3 .

. 1

15

. 1

15
3
30
5 .

uA
A
Iocp
30

uA
. 0

06
. 0

06


1
2 .


Kt

5
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:
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
Capacitance, ripple current, ESR and ESL.
Voltage rating is nominally selected to be
approximately twice the input voltage. The
RMS
assuming a low inductor ripple current (I
can be calculated from:
In general total input voltage ripple should be
maintained below 1.5% of V
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
value
the
Icin
input
R
of

2
Iout

input



capacitor
Vout
0
200
6 .
D
Copyright 2008 EXAR Corporation
capacitor
k

1 ( D
1
IN




(not to exceed
for
)
Voltage,
current,
RIP
),