Copyright
ANPEC Electronics Corp.
Rev. A.5 - Jun., 2005
APW 7093
www.anpec.com.tw
11
0
200
400
600
800
1000
1200
1400
2.8
3.3
3.8
4.3
4.8
5.3
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 V
CC
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:
SW
2
IN
D(CAP)
P
f
V
C
×
×
=
where C = 500pF and f
SW
is the switching frequency.
Resistive losses in the two power switches are
approximated by:
2
OUT
D(RES)
I
P
=
PMOS
R
×
where R
PMOS
is the on-resistance of the PMOS switch.
The junction-to-ambient thermal resistance required
to dissipate this amount of power is calculated by:
JA
= (
T
J,MAX
-
T
A,MAX
) / (
P
D
(CAP)
+ P
D
(RES)
)
where:
JA
= junction-to-ambient thermal resistance
T
J,MAX
= maximum junction temperature
T
A,MAX
= maximum ambient temperature
Application I nformation
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
R
TOFF
as a function of off-time.
4. Select the inductor as a function of output voltage,
off-time, and peak-to-peak inductor current.
Fig 3. Maximum Recommended Operation Frequency
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. V
EXTREF
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 V
IN
by connecting FB to a resistor-
divider between the output voltage and ground (Figure
5). Use 50k
for R1. R2 is given by:
=
1
V
V
R1
R2
EXTREF
OUT
V
OUT
=2.5V
V
OUT
=1.25V
V
OUT
=1.1V
V
OUT
=3.3V
Input Voltage(V)
O