ML4775
OUTPUT CAPACITOR
The choice of output capacitor is also important, as it
controls the output ripple and optimizes the efficiency of
the circuit. Output ripple is influenced by three
capacitor parameters: capacitance, ESR, and ESL. The
contribution due to capacitance can be determined by
looking at the change in capacitor voltage required to
store the energy delivered by the inductor in a single
charge-discharge cycle, as determined by the following
formula:
T
ON
×
V
IN
∆V
OUT
=
2
×
L
×
C
×
(V
OUT
−
V
IN
)
2
2
Again, a low ESR capacitor (such as tantalum) is
recommended.
DRIVING THE SHDN INPUT
Unlike other boost regulators which use external
Schottky diodes, the ML4775 has the ability to isolate
the load from the battery input when the SHDN pin is
high. Since there may be no other voltage available
when the regulator is in shutdown, the SHDN input
threshold is set well below the minimum V
IN
voltage.
SHDN can be driven directly from an open collector
device with a high value pull-up resistor to V
IN
. If SHDN
is driven from a TTL or CMOS output device, a resistor
divider should be used to prevent the SHDN input high
level from exceeding V
IN
, and to ensure the SHDN input
low level is below the 200mV threshold.
SETTING THE OUTPUT VOLTAGE
The adjustable output can be set to any voltage between
2.5V and 5.5V by connecting a resistor divider to the
SENSE pin as shown in the block diagram. The resistor
values R
1
and R
2
can be calculated using the following
equation:
V
OUT
=
0.2
×
(R
1
+
R
2
)
R
2
(4)
For a 2.4V input, and 5V output, a 27µH inductor, and
a 47µF capacitor, the expected output ripple due to
capacitor value is 87mV.
Capacitor Equivalent Series Resistance (ESR) and
Equivalent Series Inductance (ESL), also contribute to
the output ripple due to the inductor discharge current
waveform. Just after the NMOS transistor turns off, the
output current ramps quickly to match the peak inductor
current. This fast change in current through the output
capacitor’s ESL causes a high frequency (5ns) spike that
can be over 1V in magnitude. After the ESL spike settles,
the output voltage still has a ripple component equal
to the inductor discharge current times the ESR. This
component will have a sawtooth shape and a peak value
equal to the peak inductor current times the ESR. ESR
also has a negative effect on efficiency by contributing
I-squared R losses during the discharge cycle.
An output capacitor with a capacitance of 100µF, an
ESR of less than 0.1�½, and an ESL of less than 5nH is a
good general purpose choice. Tantalum capacitors
which meet these requirements can be obtained from the
following suppliers:
Matsuo
Sprague
(714) 969-2491
(603) 224-1961
(5)
The value of R
2
should be 40k�½ or less to minimize bias
current errors. R
1
is then found by rearranging the
equation:
V
R
1
=
R
2
×
OUT
−
1
0.2
(6)
It is important to note that the accuracy of these resistors
directly affects the accuracy of the output voltage. The
SENSE pin threshold variation is ±3%, and the
tolerances of R
1
and R
2
will add to this to determine the
total output variation.
In some applications, input noise may cause output
ripple to become excessive due to “pulse grouping,”
where the charge-discharge pulses are not evenly
spaced in time. In such cases it may be necessary to add
a small 20pF to 100pF ceramic feedforward capacitor
(C
FF
) from the V
IN
pin to the SENSE pin.
If ESL spikes are causing output noise problems, an EMI
filter can be added in series with the output.
INPUT CAPACITOR
Unless the input source is a very low impedance battery,
it will be necessary to decouple the input with a
capacitor with a value of between 47µF and 100µF. This
provides the benefits of preventing input ripple from
affecting the ML4775 control circuitry, and it also
improves efficiency by reducing I-squared R losses
during the charge and discharge cycles of the inductor.
LAYOUT
Good PC board layout practices will ensure the proper
operation of the ML4775. Important layout
considerations include:
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4775
• Use short trace lengths from the inductor to the V
L
pin
and from the output capacitor to the V
OUT
pin
• Use a single point ground for the ML4775 ground
pins, and the input and output capacitors
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
