APW7073 Datasheet, PDF (13/25 Pages) Anpec Electronics Coropration – Synchronous Buck PWM Controller

APW7073
Function Descriptions (Cont.)
Soft-Start/EN (Cont.)
T_{Soft −Start}
= t₂ − t₁ =
C _SS
I_SS
⋅ 2.4V
Where:
C_SS = external Soft-Start capacitor
I_SS = Soft-Start current=30uA
Voltage
VSS
4.2V
I_LIMIT
= ^{IOCSET × R OCSET}
R _{DS (ON )}
For the over-current is never occurred in the normal
operating load range; the variation of all parameters in
the above equation should be determined.
- The MOSFET’s R_DS(ON) is varied by temperature
and gate to source voltage, the user should determine
the maximum R_DS(ON) in manufacturer’s datasheet.
- The minimum I_OCSET (170uA) and minimum R_OCSET
should be used in the above equation.
- Note that the I_LIMIT is the current flow through the
upper MOSFET; I_LIMIT must be greater than maximum
output current add the half of inductor ripple current.
1.8V
VOUT
t0
t1
t2
Time
Figure 1. Soft-Start Internal
Over-Current Protection (monitor upper MOSFET)
The APW7073 provides two manners to protect the
converter from abnormal output load; one monitors
the voltage across the upper MOSFET and use the
MOSFET pin to set the over-current trip point, the other
monitors the voltage across the lower MOSFET by
comparing with an internal reference voltage (0.27V).
A resistor (R_OCSET) connected between OCSET pin and
the drain of the upper MOSFET will determine the over
current limit. An internal 200uA current source will flow
through this resistor, creating a voltage drop, which
will be compared with the voltage across the upper
MOSFET. When the voltage across the upper
MOSFET exceeds the voltage drop across the R ,
OCSET
an over-current will be detected. The threshold of the
over current limit is therefore given by:
An over current condition will shut down the device
and discharge the C_SS with a 30uA sink current and
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)
The other over-current protection monitors the output
current by using the voltage drop across the lower
MOSFET’s R and this voltage drop will be com-
DS(ON)
pared with the internal 0.27V reference voltage. If the
voltage drop across the lower MOSFET’s R_DS(ON) is
larger than 0.27V, an over-current condition is detected.
The threshold of the over current limit is given by:
I_LIMIT
= ^0.27V
R _DS(ON)
For the over-current is never occurred in the normal
operating load range; the parameters R and I
DS(ON)
LIMIT
in the above equation also have the same notices as
the previous section.
Copyright © ANPEC Electronics Corp.
13
Rev. A.1 - Apr., 2006
www.anpec.com.tw