GENERAL OVERVIEW
Overcurrent Protection
Using the above equation there is good
agreement between calculated and test results
when an RS in the range of 0.5k to 3k is used.
For RS larger than 3k test results are lower than
those predicted by (5), due to circuit parasitics.
Vin
SP6126
Gate
Q1
Using the ON/OFF Function
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 as shown on page 1.
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:
Ov er-Current Comparator
LX
-
Rs
Figure 3- Overcurrent protection circuit
The overcurrent protection circuit functions by
monitoring the voltage across the high-side FET
Q1. When this voltage exceeds 0.3V, the
overcurrent comparator triggers and the
controller enters hiccup mode. For example if
Q1 has Rds(on)=0.1 , then the overcurrent will
trigger at I = 0.3V/0.1 =3A. To program a lower
overcurrent use a resistor Rs as shown in figure
1. Calculate Rs from:
Rs
=
0.3
−
(
1.15
×
Iout
×
Rds
(
on
)
)
…..…… (5)
30
uA
The overcurrent circuit triggers at peak current
through Q1 which is usually about 15% higher
than average output current. Hence the
multiplier 1.15 is used in (5).
Example: A switching MOSFET used with
SP6126 has Rds(on) of 0.1 . Program the over-
current circuit so that maximum output is 2A.
Rs
=
0.3
−
(
1.15
×
1
A
×
0.1
Ohm
)
30
uA
Rs = 2333
Nov07-08
RevG
+
30uA
Vin - 0.3V
Ds
R
2
=
R
1
Vout
Vref
−
1
Where: Vref=0.6 is the reference voltage of the
SP6126
R1=200k
is a fixed-value resistor that, in
addition to being a voltage divider, it is part of
the compensation network. In order to simplify
compensation calculations, R1 is fixed at 200k .
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.
MOSFET Gate Drive
P-channel drive is derived through an internal
regulator that generates V
IN
-5V. This pin (VDR)
has to be connected to V
IN
with a 0.1uF
decoupling capacitor. The gate drive circuit
swings between V
IN
and V
IN
-5 and employs
powerful drivers for efficient switching of the P-
channel MOSFET.
SP6126: TSOT-6 PFET Buck Controller
2008 Exar Corporation
6
EXAR [ EXAR CORPORATION ]
