SP6127 データシートの表示（PDF） - Signal Processing Technologies
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SP6127 Datasheet PDF : 12 Pages
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______________________________________________________ GENERAL OVERVIEW
Overcurrent Protection (OCP)
Vin
SP6127
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 the maximum value of Rs should be
limited to 3kΩ.
Gate
Q1
Ov er-Current Comparator
LX Rs
Ds
30uA
Vin - 0.3V
Note that in order to safeguard against false
overcurrent trigger due to transients, there is a
150ns delay between the turn on of the MOSFET
and when OCP circuit is activated. As a
consequence at very high Vo/VIN ratio, where
MOSFET on-time is less than 150ns, the OCP
circuit will not detect overcurrent.
Figure 3 Overcurrent protection circuit
The overcurrent protection (OCP) 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) × Kt ) ...(5)
30uA
Where:
1.15 is used to calculate peak inductor current
which is nominally 15% higher than average
output current
RDS(ON) is MOSFET ON-resistance rating
Kt is a multiplier that accounts for increase in
RDS(ON) due to temperature
Example: A switching MOSFET used with
SP6127 has RDS(ON) of 0.08Ω and Kt is 1.5.
Program the over-current circuit so that
maximum output is 2A.
Rs = 0.3 − (1.15 × 2A × 0.08Ohm ×1.5)
30uA
Rs=800Ω
Using the above equation there is good
agreement between calculated and test results
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 SP6127
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.
June 26, 2007
SP6127 TSOT-6 PFET Buck Controller
Page 6
2007 Sipex Corporation
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