APW7074 Ver la hoja de datos (PDF) - Anpec Electronics
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APW7074 Datasheet PDF : 20 Pages
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APW7074
Function Descriptions (Cont.)
Over-Current Protection (Cont.)
- The MOSFET’RDS(ON) is varied by temperature
and gate to source voltage, the user should determine
the maximum RDS(ON) in manufacturer’s datasheet.
- The minimum IOCSET (170uA) and minimum ROCSET
should be used in the above equation.
- Note that the ILIMIT is the current flow through the
upper MOSFET; ILIMIT must be greater than maximum
output current add the half of inductor ripple current.
An over current condition will shut down the device
and discharge the C with a 10uA sink current and
SS
then initiate the soft-start sequence. If the over current
condition is not removed during the soft-start interval,
the device will be shut down while the over current is
detected and the SS still rises to 4V to complete its
cycle. The soft start function will be cycled until the
over current condition is removed. Both over-current
protections have the same behavior while an over
current condition is detected.
Over-Current Protection (monitor lower MOSFET)
MOSFET’s RDS(ON) and this voltage drop will be com-
pared with the internal 0.27V reference voltage. If the
voltage drop across the lower MOSFET’s R is
DS(ON)
larger than 0.27V, an over-current condition is detected.
The threshold of the over current limit is given by:
ILIMIT
= 0.27V
R DS(ON)
For the over-current is never occurred in the normal
operating load range; the parameters RDS(ON) and ILIMIT
in the above equation also have the same notices as
the previous section.
Under Voltage Protection
The FB pin is monitored during converter operation by
their own Under Voltage (UV) comparator. If the FB
voltage drops below 50% of the reference voltage (50%
of 0.8V = 0.4V), a fault signal is internally generated,
and the device turns off both high-side and low-side
MOSFET and the converter’s output is latched to be
floating.
The other over-current protection monitors the output
current by using the voltage drop across the lower
Application Information
Output Voltage Selection
Output Inductor Selection
The output voltage can be programmed with a resistive
divider. Use 1% or better resistors for the resistive
divider is recommended. The FB pin is the inverter
input of the error amplifier, and the reference voltage
is 0.8V. The output voltage is determined by:
VOUT
= 0.8 × 1+
R OUT
R GND
Where R is the resistor connected from V to FB
OUT
OUT
and R is the resistor connected from FB to GND.
GND
The inductor value determines the inductor ripple
current and affects the load transient response. Higher
inductor value reduces the inductor’s ripple current and
induces lower output ripple voltage. The ripple current
and ripple voltage can be approximated by:
IRIPPLE
=
VIN − VOUT
FS × L
× VOUT
VIN
∆VOUT = IRIPPLE × ESR
Copyright © ANPEC Electronics Corp.
12
Rev. A.2 - Feb., 2006
www.anpec.com.tw
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