ADP2140
Data Sheet
Rev. A | Page 24 of 32
CH1 500mV
CH2 500mV
M40.0祍
A CH3 1.16V
T 10.00%
CH3 2.00V
T
LDO OUTPUT
BUCK OUTPUT
EN1
Figure 84. Individual Activation Mode, EN1 and EN2 Pins Tied Together
POWER-GOOD FUNCTION
The ADP2140 power-good (PG) pin indicates the state of the
monitored output voltages. The PG function is the logical AND
of the state of both outputs. The PG function is an active high,
open-drain output, requiring an external pull-up resistor typically
supplied from the I/O supply rail, as shown in . When the sensed
output voltages are below 92% of their nominal value, the PG pin is
held low. When the sensed output voltages rise above 92% of
the nominal levels, the PG line is pulled high after t
RESET
. The
PG pin remains high as long as the sensed output voltages are
above 86% of the nominal output voltage levels.
The typical PG delay when the buck is in PWM mode is 5 ms.
When the part is in PSM mode, the PG delay is load dependent
because the internal clock is disabled to reduce quiescent current
during the sleep stage. PG delay varies from hundreds of micro-
seconds at 10 mA, up to seconds at current loads of less than 10 糀.
CH1 2.00V
CH2 2.00V
CH4 2.00V
M2.00ms
A CH1 2.20V
T 10.20%
CH3 2.00V
T
EN1
BUCK
LDO
PG
Figure 85. Typical PG Timing
EXTERNAL COMPONENT SELECTION
The external component selection for the ADP2140 application
circuit that is shown in Table 8, Table 9, and Figure 86 is dependent
on input voltage, output voltage, and load current requirements.
Additionally, trade-offs between performance parameters such
as efficiency and transient response can be made by varying the
choice of external components.
SELECTING THE INDUCTOR
The high frequency switching of the ADP2140 allows the selection
of small chip inductors. The inductor value affects the transi-
tion between CFM to PSM, efficiency, output ripple, and current
limit values. Use the following equation to calculate the inductor
ripple current:
L
f
V
V
V
V
I
sw
IN
OUT
IN
OUT
L
?/DIV>
?/DIV>
?/DIV>
=
)
(
where:
f
SW
is the switching frequency (3 MHz typical).
L is the inductor value.
The dc resistance (DCR) value of the selected inductor affects
efficiency, but a decrease in this value typically means an increase
in root mean square (rms) losses in the core and skin. As a
minimum requirement, the dc current rating of the inductor
should be equal to the maximum load current plus half of the
inductor current ripple, as shown by the following equation:
)
2
?/DIV>
(
)
(
L
MAX
LOAD
PK
I
I
+
=
OUTPUT CAPACITOR
Output capacitance is required to minimize the voltage over-
shoot and ripple present on the output. Capacitors with low
equivalent series resistance (ESR) values produce the lowest
output ripple; therefore, use capacitors such as the X5R dielectric.
Do not use the Y5V and Z5U capacitors; they are not suitable
for this application because of their large variation in capacitance
over temperature and dc bias voltage. Because ESR is important,
select the capacitor using the following equation:
L
RIPPLE
COUT
I
V
SR
?/DIV>
d
where:
ESR
COUT
is the ESR of the chosen capacitor.
V
RIPPLE
is the peak-to-peak output voltage ripple.
Use the following equations to determine the output
capacitance:
RIPPLE
SW
IN
OUT
V
L
f
?/DIV>
V
?/DIV>
?/DIV>
?/DIV>
e
2
)
2
(
OUT
SW
L
OUT
V
f
I
?/DIV>
8
?/DIV>
?/DIV>
?/DIV>
e
Increasing the output capacitor has no effect on stability and
increasing the output capacitance may further reduce output
ripple and enhance load transient response. When choosing this
value, it is also important to account for the loss of capacitance
due to output voltage dc bias.
INPUT CAPACITOR
Input capacitance is required to reduce input voltage ripple; there-
fore, place the input capacitor as close as possible to the VINx
pins. As with the output capacitor, a low ESR X7R- or X5R-type
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