APW7093 查看數據表（PDF） - Anpec Electronics
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APW7093 Datasheet PDF : 20 Pages
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APW7093
Function Descriptions(Cont.)
Shutdown
Drive SHDN to a logic-level low to place the APW7093
in low-power shutdown mode and reduce supply
current less than 1µA. In shutdown, all circuitry and
internal MOSFETs turn off, so the LX node becomes
high impedance. Drive SHDN to a logic-level high or
connect to VCC for normal operation.
Power Dissipation
Power dissipation in the APW7093 is dominated by
conduction losses in the two internal power switches.
Power dissipation due to charging and discharging the
gate capacitance of the internal switches (i.e., switching
losses) is approximately:
PD(CAP) = C × VIN2 × fSW
where C = 500pF and fSW is the switching frequency.
Resistive losses in the two power switches are
approximated by:
PD(RES) = I2OUT × RPMOS
where RPMOS is the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required
to dissipate this amount of power is calculated by:
θJA = (TJ,MAX - TA,MAX) / (PD(CAP) + PD(RES))
where:
θ JA = junction-to-ambient thermal resistance
TJ,MAX = maximum junction temperature
TA,MAX = maximum ambient temperature
Application Information
For typical applications, use the recommended
component values in Figure 2. For other applications,
take the following steps:
1. Select the desired PWM-mode switching frequency.
See Figure 3 for maximum operating frequency.
2. Select the constant off-time as a function of input
voltage, output voltage, and switching frequency.
3. Select RTOFF as a function of off-time.
4. Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
1400
1200
VOUT=2.5V
1000
VOUT=3.3V
800
600
400
VOUT=1.1V
200
VOUT=1.25V
Setting the Output Voltage
An external voltage applied to the EXTREF pin sets
the output voltage of the APW7093. This can come
directly from another voltage source or external
reference. When FB is directly tied to the output
(Figure 4), the output voltage range is limited by the
external reference’s input voltage limits. VEXTREF
should be limited to less than VIN-1.7V. Failure to
comply can cause the part to operate abnormally and
may cause part damage. Alternatively, the output can
be adjusted up to VIN by connecting FB to a resistor-
divider between the output voltage and ground (Figure
5). Use 50k Ω for R1. R2 is given by:
R2
=
R1⋅
VOUT
VEXTREF
− 1
0
2.8 3.3 3.8 4.3 4.8 5.3
Input Voltage(V)
Fig 3. Maximum Recommended Operation Frequency
Copyright © ANPEC Electronics Corp.
11
Rev. A.5 - Jun., 2005
www.anpec.com.tw
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