ML4790
The boost capacitor‘s Equivalent Series Resistance (ESR)
and Equivalent Series Inductance (ESL), also contribute to
the 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 boost capacitor‘s ESL
causes a high frequency (5ns) spike that can be over 1V in
magnitude. After the ESL spike settles, the boost voltage
still has a ripple component equal to the inductor
discharge current times the ESR. This component will have
a sawtooth waveshape and can be calculated using the
following formula:
ESR
≤
∆
V
BOOST
(in
Ω
)
I
L(PEAK )
SETTING THE OUTPUT VOLTAGE
The adjustable output can be set to any voltage between
2.5V and 5.5V 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)
The value of R
2
should be 40kΩ or less to minimize bias
current errors. R
1
is then found by rearranging the
equation:
(4)
For example, a 2.4V input, a 22µH inductor, and an
allowance of 100mV of ripple on the boost capacitor
results in a maximum ESR of 200mΩ. Therefore, a boost
capacitor with a capacitance of 22µF or 33µF, an ESR of
less than 200mΩ, and an ESL of less than 5nH is a good
choice. Tantalum capacitors which meet these
requirements can be obtained from the following
suppliers:
AVX
Sprague
OUTPUT CAPACITOR
The LDO stage output capacitor (C1) is required for
stability and to provide a high frequency filter. An output
capacitor with a capacitance of 100µF, an ESR of less than
100mΩ, and an ESL of less than 5nH is a good general
purpose choice.
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 filtering
prevents the input ripple from affecting the ML4790
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.
(207) 282-5111
(207) 324-4140
V
R
1
=
R
2
×
OUT
−
1
0.2
(6)
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.
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 500pF to 1000pF ceramic
feedback capacitor (C
FB
) from the V
OUT
pin to the SENSE
pin.
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
