ML4961
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:
∆V
OUT
T
ON
×
V
IN
=
2
×
L
×
C
×
(V
OUT
−
V
IN
)
2
2
The value of R
2
should be 40kΩ 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)
(4)
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.
SETTING THE
RESET
THRESHOLD
To use the
RESET
comparator as an input voltage monitor,
it is necessary to use an external resistor divider tied to the
DETECT pin as shown in the block diagram. The resistor
values R
A
and R
B
can be calculated using the following
equation:
V
IN(MIN)
=
0.2
×
(R
A
+
R
B
)
R
B
(7)
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 0.1Ω, 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:
AVX
Sprague
(207) 282-5111
(207) 324-4140
The value of R
B
should be 100kΩ or less to minimize bias
current errors. R
A
is then found by rearranging the
equation:
V
R
A
=
R
B
×
IN(MIN)
−
1
0.2
(8)
LAYOUT
Good PC board layout practices will ensure the proper
operation of the ML4961. Important layout considerations
include:
• Use adequate ground and power traces or planes
• Keep components as close as possible to the ML4961
• 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 ML4961 ground pins,
and the input and output capacitors
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
ML4961 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.
SETTING THE OUTPUT VOLTAGE
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
1
and R
2
can be calculated using the following equation:
V
OUT
=
0.2
×
(R
1
+
R
2
)
R
2
(5)
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
