Rainbow Electronics MAX863 User Manual
Page 11
MAX863
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
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11
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.
Determine the Peak Switching Current
(Graphical Method)
The peak switching current set by R
SENSE
determines
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.
Determine the Peak Switching Current and
Inductance (Analytical Method)
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,
ξ
MIN
(Figure 6):
where t
OFF(MIN)
= 2µs and t
ON(MAX)
= 17.5µs.
Select a value for
ξ greater than ξ
MIN
. If
ξ
MIN
is less
than 1, an acceptable choice is (
ξ
MIN
+ 1) / 2. If
ξ
MIN
is
greater than 1, values between
ξ
MIN
and 2 x
ξ
MIN
are
acceptable (1.5 x
ξ
MIN
, for example). Values greater
than 1 represent designs with full-load operation in dis-
continuous-conduction mode.
Now calculate the peak switching current and induc-
tance. If
ξ
MIN
≤ ξ ≤ 1, use:
For
ξ ≥ 1%, use:
The suggested inductor value is:
Round L up to the next standard inductor value.
Choose R
SENSE
The peak switching current is set by R
SENSE
(R1 and
R2 in Figure 2):
Verify that you’ve selected the correct R
SENSE
by test-
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
SENSE
to the next lowest value.
The current-sense resistor’s power rating should be
higher than:
R
V
R
POWER RATING
2
CS MAX
SENSE
=
( )
R
V
I
SENSE
CS MIN
PEAK
≤
=
( )
85mV
I
PEAK
L
V
- V
x t
I
x
OUT
IN MIN
OFF MIN
PEAK
≅
( )
( )
ξ
I
= 2 x I
x
V
+ V
x
PEAK
IN,DC MAX
OUT
IN
( )
−
( )
ξ 1
V
OUT
I
= I
x
2
2 -
PEAK
IN,DC MAX
( )
ξ
ξ
MIN
OFF MIN
ON MAX
OUT
IN MIN
IN MIN
t
t
x
V
V
V
=
−
( )
( )
( )
( )
I
V
x I
0.8 x V
IN,DC MAX
OUT
OUT
IN MIN
( )
( )
≅
INDUCTOR CURRENT, I
L
t
∆I
L
ξ
MIN
= ∆
I
L
I
PEAK
I
PEAK
Figure 6. Ratio of Inductor AC Ripple Current to Peak Current