APW7093NI-TRL 데이터 시트보기 (PDF) - Anpec Electronics
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APW7093NI-TRL Datasheet PDF : 20 Pages
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APW7093
Application Information(Cont.)
Setting the Output Voltage (Cont.)
( ( ) ) TOFF
=
VIN − VOUT − VPMOS
fSW VIN − VPMOS + VNMOS
LX
APW7093
VOUT=VEXTREF where:
TOFF = the programmed off-time
VIN = the input voltage
VOUT = the output voltage
VPMOS = the voltage drop across the internal PMOS
VEXTREF
EXTREF FB
power switch |IOUT X RPMOS|
VNMOS = the voltage drop across the internal NMOS
synchronous-rectifier switch |IOUT X RNMOS|
1.1V ≤ VEXTREF ≤ VIN − 1.7V
FIG.4 Adjsting the Output Voltage using EXTREF
fSW = switching frequency
Make sure that TON and TOFF are greater than 400ns
when sourcing current. Select RTOFF according to the
formula:
VOUT
R TOFF = (TOFF − 0.18 µs) × (109k Ω/1.00 µs)
LX
APW7093
EXTREF
FB
REF
R2
=
R1 ⋅
VOUT
VEXTREF
− 1
Recommended values for RTOFF range from 24k Ω to
R2
410k Ω for off-times of 0.4µs to 4µs. Often the switching
frequency is set as high as possible, and the inductor value
is reduced to minimize the energy transferred from
inductor to capacitor during load-step recovery.
The operating frequency of the APW7093 is determined
R1
primarily by TOFF (set by RTOFF), VIN, and VOUT as
shown in the following formula:
( ( ) ) fSW
=
VIN − VOUT − VPMOS
TOFF VIN − VPMOS + VNMOS
where VEXTREF = VREF = 1.1V
FIG.5 Adjsting the Output Voltage using FB
Programming the Switching Frequency and
Off-Time and On-Time
The APW7093 features a programmable PWM-mode
switching frequency, which is set by the input and
output voltage and the value of RTOFF, connected from
TOFF to GND. RTOFF sets the PMOS power switch
off-time in PWM mode. Use the following equation to
select the off-time while sourcing current according to
the desired switching frequency in PWM mode:
However, as the output current increases, the voltage
drop across the NMOS and PMOS switches increases
and the voltage across the inductor decreases. This
causes the frequency to drop. Assuming RPMOS =
RNMOS, the change in frequency can be approximated
with the following formula:
∆fSW
=
− ∆IOUT ×RPMOS
VIN × TOFF
where RPMOS is the resistance of the internal
MOSFETs (70m Ω typ).
Copyright © ANPEC Electronics Corp.
12
Rev. A.5 - Jun., 2005
www.anpec.com.tw
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