MAX863 Maxim, MAX863 Datasheet - Page 11

• Manufacturer Part Number: MAX863
• Description: Dual / High-Efficiency / PFM / Step-Up DC-DC Controller
• Manufacturer: Maxim

minimum input voltages. Estimate the maximum input
currents for each output based on the minimum input
voltage and desired output power:
where 0.8 is chosen as a working value for the nominal
efficiency. The power source must be capable of deliv-
ering the sum of the maximum input currents of both
DC-DC converters.
The peak switching current set by R
the amount of energy transferred from the input on
each cycle. For 3.3V, 5V, 12V, and 24V output circuits,
the peak current can be selected using the output cur-
rent curves shown in Figures 5a–5d.
The following boost-circuit equations are useful when
the desired output voltage differs from those listed in
Figure 5. They allow trading off peak current and induc-
tor value in consideration of component availability,
size, and cost.
Begin by calculating the minimum allowable ratio of
inductor AC ripple current to peak current,
(Figure 6):
where t
Select a value for
than 1, an acceptable choice is (
greater than 1, values between
acceptable (1.5 x
than 1 represent designs with full-load operation in dis-
continuous-conduction mode.
Now calculate the peak switching current and induc-
tance. If
For ≥ 1%, use:
Determine the Peak Switching Current and
I
PEAK
Determine the Peak Switching Current
OFF(MIN)
MIN
MIN
= 2 x I
I
IN,DC MAX
I
PEAK
≤ ≤ 1, use:
Inductance (Analytical Method)
= 2µs and t
IN,DC MAX
t
t
OFF MIN
ON MAX
______________________________________________________________________________________
= I
MIN
greater than
IN,DC MAX
, for example). Values greater
V
0.8 x V
x
ON(MAX)
x
OUT
V
Dual, High-Efficiency, PFM, Step-Up
V
(Graphical Method)
OUT
OUT
MIN
x I
V
MIN
IN MIN
x
IN MIN
MIN
OUT
2 -
+ V x
= 17.5µs.
SENSE
V
V
2
+ 1) / 2. If
and 2 x
IN MIN
OUT
. If
IN
determines
MIN
MIN
1
is less
MIN
MIN
are
is
The suggested inductor value is:
Round L up to the next standard inductor value.
The peak switching current is set by R
R2 in Figure 2):
Verify that you’ve selected the correct R
ing the prototype using the minimum input voltage
while supplying the maximum output current. If the out-
put voltage droops, then decrease the value of the cur-
rent-sense resistor and adjust the other components as
necessary.
The current-sense resistor must be a small, low-induc-
tance type such as a surface-mount metal-strip resistor.
Do not use wire-wound resistors, since their high induc-
tance will corrupt the current feedback signal. In order
to allow use of standard resistor values, round R
to the next lowest value.
The current-sense resistor’s power rating should be
higher than:
Figure 6. Ratio of Inductor AC Ripple Current to Peak Current
L
R
R
POWER RATING
SENSE
DC-DC Controller
V
MIN
OUT
=
I
I
PEAK
L
I
L
- V
V
CS MIN
IN MIN
I
I
PEAK
PEAK
t
x
V
2
R
x t
CS MAX
SENSE
OFF MIN
I
85mV
I
PEAK
PEAK
Choose R
SENSE
SENSE
(R1 and
by test-
SENSE
SENSE
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