SP6125 Ver la hoja de datos (PDF) - Signal Processing Technologies
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SP6125 Datasheet PDF : 12 Pages
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General Overview
Overcurrent protection
SP6125
Gate
Vin
Q1
Ov er-Current Comparator
LX Rs
Ds
30uA
Vin - 0.3V
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)=0.1Ω, then the overcurrent will
trigger at I = 0.3V/0.1Ω=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)
30uA
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
SP6125 has Rds(on) of 0.1Ω. Program the over-
current circuit so that maximum output is 2A.
Rs = 0.3 − (1.15 × 2A × 0.1Ohm)
30uA
Rs = 2333Ω
Using the above equation there is good
agreement between calculated and test results
for Rs in the range of 0.5kΩ to 3kΩ. For Rs
larger than 3kΩ test results are lower than those
predicted by (5), due to circuit parasitics.
Therefore maximum value of Rs should be
limited to 3kΩ.
Using the ON/OFF Function
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 the 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
10kΩ 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:
Where:
R2
=
R1
Vout
Vref
− 1
Vref=0.6 is the reference voltage of the SP6125
R1=200kΩ 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
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.
MOSFET Gate Drive
P-channel drive is derived through an internal
regulator that generates VIN-5V. This pin (VDR)
has to be connected to VIN with a 0.1uF
decoupling capacitor. The gate drive circuit
swings between VIN and VIN-5 and employs
powerful drivers for efficient switching of the P-
channel MOSFET.
Mar22-07 RevC
SP6125: TSOT-6 PFET Buck Controller
6
2007 Sipex Corporation
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