ML4880
BUCK REGULATORS — INPUT CAPACITOR SELECTION
The choice of the input capacitor is based on it’s ripple
current and voltage ratings rather than its capacitance
value. The input capacitor should be a low ESR type and
located as close to the source of the P-MOS switch as
possible. The input capacitor’s ripple current is
determined by the load current and the input voltage, with
the worst case condition occurring at V
IN
= 2
×
V
OUT
:
(V
−
V
OUT
)
V
I
RMS(C )
≈
I
SENSE(MAX)
+
1
∆
I
L(MAX)
OUT IN
(13)
IN
2
V
IN
The selected capacitor must meet both the capacitance
and ESR requirements. As a final check, make sure the
output capacitor can handle the ripple current,
I
RMS(COUT)
:
I
RMS(C
OUT
)
≈
∆
I
L(MAX)
12
(16)
BUCK REGULATORS - OUTPUT VOLTAGE
The output of buck regulator A is adjustable and can be
set to any voltage between 2.5V and 3.5V by connecting a
resistor divider to the feedback pin as shown in Figure 1.
Similarly, the output voltage of buck regulator B is
adjustable and has a usable range of 4.5V to 5.0V. The
resistor values R1 and R2 can be calculated using the
following equation:
The capacitor’s voltage rating is based on the maximum
input voltage, V
IN(MAX)
. Capacitor manufacturers typically
recommend derating the capacitor voltage rating by 20%
to 50% for aluminum electrolytic types and 50% to 70%
for tantalum types.
In high current applications it may necessary to add a
10µF bulk capacitor and a small 0.1µF ceramic capacitor
to bypass V
IN
(pin 16) right at the ML4880.
BUCK REGULATORS — OUTPUT CAPACITOR
SELECTION
The output capacitors determine the loop stability and the
output ripple voltage. Use only low ESR capacitors
intended for switching power supply applications, such as
AVX TPS, Sprague 593D Sanyo OS-CON, or Nichicon PL
series. To ensure stability, the minimum capacitance value
is given by:
C
OUT
T
≥
4.3
ON(MAX)
V
OUT
R
SENSE
V
OUT
=
1.25V
×
R1
+
R2
R2
(17)
The value of R2 should be 125kΩ or less to minimize bias
current errors.
It is important to note that the accuracy of these resistors
directly affects the accuracy of the output. Use precision
resistors and set the nominal voltage approximately 1% to
2% high with no load in order to make up for the drops
that normally occur when supplying heavy loads. This
offset results in the best overall output accuracy over line
and load.
FLYBACK REGULATOR — INDUCTOR AND SENSE
RESISTOR SELECTION
Figure 7 shows the inductor current of the flyback
regulator. The inductor current is made up of two
components: the DC current level set by the
transconductance amplifier, I
SENSE
, and the inductor ripple
current,
∆I
L
. Therefore, the selection of the inductance
value determines how much of the output current is made
up of the inductor ripple current. Higher inductor ripple
current allows smaller inductor values, but results in
higher peak currents, lower efficiency, and higher output
voltage ripple.
(14)
The maximum ESR value can be estimated using:
ESR
≤
∆
V
OUT
∆
I
L(MAX)
(15)
I
L
V
IN
L
V
OUT
L
∆I
L(PEAK)
∆I
L
=
V
IN
T
ON
L
V
SENSE
I
SENSE
=
R
SENSE
I
OUT
= (I
SENSE
+ 1/2
∆I
L
)
V
IN
V
IN
+ V
OUT
T
ON
T
OFF
t
Figure 7. Flyback Regulator Inductor Current
9
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
