ML4851
DESIGN CONSIDERATIONS
(Continued)
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
2
R losses during the discharge cycle.
An output capacitor with a capacitance of 100µF, an ESR
of less than 0.1W, 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
(207) 282-5111
(207) 324-4140
After the appropriate inductor value is chosen, it is
necessary to find the minimum inductor current rating
required. Peak inductor current is determined from the
following formula:
I
L(PEAK )
=
t
ON( MAX )
×
V
IN( MAX )
L
MIN
(3)
In the two cell application previously described, a
maximum input voltage of 3V would give a peak current
of 1A. When comparing various inductors, it is important
to keep in mind that suppliers use different criteria to
determine their ratings. Many use a conservative current
level, where inductance has dropped to 90% of its normal
level. In any case, it is a good idea to try inductors of
various current ratings with the ML4851 to determine
which inductor is the best choice. Check efficiency and
maximum output current, and if a current probe is
available, look at the inductor current to see if it looks
like the waveform shown in Figure 3. For additional
information, see Applications Note 29, “Choosing an
Inductor for Your ML4861 Application.”
Suitable inductors can be purchased from the following
suppliers:
Coilcraft
Coiltronics
Dale
Sumida
(847) 639-6400
(561) 241-7876
(605) 665-9301
(847) 956-0666
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 ML4851 control circuitry, and it also
improves efficiency by reducing I
2
R losses during the
charge and discharge cycles of the inductor. Again, a low
ESR capacitor (such as tantalum) is recommended.
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
REF
) with a
small 10nF to 100nF ceramic capacitor. This is
particularly true if the ripple voltage at V
IN
is greater than
100mV.
SETTING THE
RESET
THRESHOLD
To use the
RESET
comparator as an input voltage monitor,
as shown in Figure 1, 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:
XFMRS, Inc. (317) 834-1066
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
1
2
6
(4)
For a 2.4V input, and 5V output, a 18µH inductor, and a
47µF capacitor, the expected output ripple due to
capacitor value is 33mV.
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
V
IN( MIN)
=
0.2
×
1
R
+
R
6
A
B
R
B
(5)
The value of R
B
should be 100kW or less to minimize bias
current errors. R
A
is then found by rearranging the
equation:
DATASHEET
July 2000
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
