ML4851
DESIGN CONSIDERATIONS (Continued)
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:
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
t
ON(MAX) × V
IN(MAX)
(3)
IL(PEAK)
=
2
I R losses during the discharge cycle.
LMIN
In the two cell application previously described, a
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:
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 forYour ML4861 Application.”
Matsuo
(207) 282-5111
(207) 324-4140
Sprague
If ESL spikes are causing output noise problems, an EMI
filter can be added in series with the output.
INPUTCAPACITOR
Suitable inductors can be purchased from the following
suppliers:
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
Coilcraft
Coiltronics
Dale
(847) 639-6400
(561) 241-7876
(605) 665-9301
(847) 956-0666
2
improves efficiency by reducing I R losses during the
charge and discharge cycles of the inductor. Again, a low
ESR capacitor (such as tantalum) is recommended.
Sumida
REFERENCE CAPACITOR
XFMRS, Inc. (317) 834-1066
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
OUTPUTCAPACITOR
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:
be necessary to decouple the reference pin (V ) with a
REF
small 10nF to 100nF ceramic capacitor. This is
particularly true if the ripple voltage at V is greater than
IN
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
2
tON2 × V
IN
∆VOUT
=
(4)
2×L × C × V − V
1
IN6
OUT
block diagram. The resistor values R and R can be
A
B
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.
calculated using the following equation:
R +R
1 6
A
B
V
= 0.2×
IN(MIN)
(5)
RB
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
The value of R should be 100kW or less to minimize bias
B
current errors. R is then found by rearranging the
A
equation:
DATASHEET
July 2000
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
