ML4761
OUTPUT CAPACITOR
SETTING THE OUTPUT VOLTAGE
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:
The adjustable output can be set to any voltage between
2.5V and 6V by connecting a resistor divider to the
SENSE pin as shown in the block diagram. The resistor
values R and R can be calculated using the following
1
2
equation:
(R +R )
1
2
V
= 0.2 ×
(5)
OUT
R
2
The value of R should be 40ký or less to minimize bias
2
2
TON2 × V
current errors. R is then found by rearranging the
1
IN
∆VOUT
=
(4)
equation:
2 ×L × C × (VOUT − V )
IN
V
0.2
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.
OUT
R = R ×
−1
(6)
1
2
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
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.
of R and R will add to this to determine the total output
1
2
variation.
REFERENCE CAPACITOR
Under some circumstances input ripple cannot be
reduced effectively. This occurs primarily in applications
where inductor currents are high, causing excess output
ripple due to “pulse grouping”, where the charge-
discharge pulses are not evenly spaced in time. In such
cases it may be necessary to decouple the reference pin
(V ) with a small 10nF to 100nF ceramic capacitor. This
REF
is particularly true if the ripple voltage at V is greater
IN
An output capacitor with a capacitance of 100µF, an ESR
of less than 0.1ý, 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:
than 100mV.
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
AVX
(207) 282-5111
(207) 324-4140
100pF ceramic feedforward capacitor (C ) from the V
FF
IN
pin to the SENSE pin.
Sprague
If ESL spikes are causing output noise problems, an EMI
filter can be added in series with the output.
LAYOUT
Good PC board layout practices will ensure the proper
operation of the ML4761. Important layout considerations
include:
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
ML4761 control circuitry, and it also improves efficiency
by reducing I-squared R losses during the charge and
discharge cycles of the inductor. Again, a low ESR
capacitor (such as tantalum) is recommended.
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4761
• Use short trace lengths from the inductor to the V pin
L
and from the output capacitor to the V
pin
OUT
• Use a single point ground for the ML4761 ground pins,
and the input and output capacitors
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
