ML4875
ML4875-5.0
100
L = 56µH
90
EFFICIENCY (%)
90
L = 56µH
L = 27µH
80
L = 15µH
70
100
ML4875-3.3
80
L = 15µH
70
60
EFFICIENCY (%)
L = 27µH
60
50
0
1.0
2.0
3.0
V
IN
(V)
4.0
5.0
50
0
1.0
2.0
V
IN
(V)
3.0
ML4875-3.0
100
90
EFFICIENCY (%)
L = 56µH
L = 27µH
80
70
L = 15µH
60
50
0
1.0
2.0
V
IN
(V)
3.0
Figure 5. Typical Efficiency as a Function of V
IN
.
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
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
(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
If ESL spikes are causing output noise problems, an EMI
filter can be added in series with the output.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
