Datasheet

MAX8686
Setting the Peak Current Limit
The peak current-limit threshold (V
CS+
- V
CS-
) is set by
a resistor connected from ILIM to GND. An internal
10µA current source flows through this resistor to set a
voltage that is 61 times higher than the current-limit
threshold. For example, a 300kΩ resistor sets the cur-
rent-limit threshold at (10µA x 300kΩ)/61 or 49mV:
where R
ILIM
is in kilohms, V
TH
is in millivolts, and corre-
sponds to the peak voltage across the sensing element
(inductor resistance or current-sense resistor).
This allows a maximum average DC output current of
(ILIM):
where R
DC
is the DC resistance of the inductor or sense
resistor and I
P-P
is the peak-to-peak inductor current.
To ensure maximum output current, use the minimum
value of V
TH
from each setting and the maximum R
DC
values at the highest expected operating temperature.
The DC resistance of the inductor’s copper wire has a
+0.38%/°C temperature coefficient.
When using a sense resistor, the current through the
sense resistor sets a voltage compared with the peak
current limit.
To provide a more efficient and lower cost design, the
current can be measured through the inductor using a
DCR method (voltage across the DC resistance of the
inductor) as shown in Figure 5.
An RC circuit is connected across the inductor. The RC
time constant is set to be 1.1 to 1.2 times the inductor
time constant (L/R
DC
). Pick the value of C1 in the 1µF to
4.7µF range, and then calculate R1 from:
R2 is added in some applications to scale down the
current signal. R2 and LIR should be selected to meet
the following condition.
Calculating the Phase Voltage
In the multiphase converter, the phases are interleaved
to reduce the output voltage ripple. The master starts
conduction at the beginning of the FREQ ramp. The
phase delay time, t
PHASE
, is the conduction delay time
of slaves from the master. Determine the phase delay
time as follows:
where X is the number of the slave (X = 1 to 5 for 6
phase converters) f
SW
is the switching frequency per
phase in kilohertz, and N is the total number of phases.
Calculate the phase voltage of each slave from:
where C
FREQ
is the total capacitance (in picofarads) at
FREQ (see the
Setting the Switching Frequency
sec-
tion). For better jitter immunity, V
PHASE
should be limit-
ed between 0.3V and 2.5V.
Then determine resistor-divider for each slave.
Preselect more than 10kΩ for phase resistor RX5 (X = 2
to 6, R25, R35, R45, R55, and R65) in Figure 4, and cal-
culate RX4 (R24, R34, R44, R54, and R64) as follows:
Input Capacitor
The input capacitor reduces the peak current drawn
from the power source and reduces the noise and volt-
age ripple on the input DC voltage bus caused by the
circuit’s switching. The input capacitors must meet the
RX RX
VV
V
PHASE X
PHASE X
45
54
=×
−.
()
()
V
txx
C
PHASEX
PHASEX
FREQ
=
−510 30
8
t
X
fx xN
PHASEX
SW
=
10
3
I
N
x
LIR
xR x
R
RR
OUT MAX
DC
_
1
2
2
1
+
⎛
⎝
⎜
⎞
⎠
⎟
+
22
45≤ mV
R
xL
RxC
DC
1
12 1
1
.
=
I
V
R
I
LIM
TH
DC
PP
= −
−
2
VV V
xR
TH CS CS
ILIM
= − =
+ −
10
61
Single/Multiphase, Step-Down,
DC-DC Converter Delivers Up to 25A Per Phase
18 ______________________________________________________________________________________
MAX8686
V
OUT
L1
C1
R1
R2
LX
CS+
CS-
Figure 5. Current Sense Using the Inductor’s DC Resistance