MAX1887 Maxim Integrated Products, MAX1887 Datasheet - Page 20

• Manufacturer Part Number: MAX1887
• Description: (MAX1887 / MAX1897) Quick-PWM Slave Controllers
• Manufacturer: Maxim Integrated Products

current-limit threshold. This is acceptable since the
slave’s inductor current limit only serves as a fail-safe in
case the master and slave inductor currents become
significantly unbalanced during a transient.
The basic operating conditions are determined using
the same calculations provided in any Quick-PWM reg-
ulator data sheet. The valley of the inductor current
(I
number of phases minus half of the peak-to-peak
inductor current:
where the peak-to-peak inductor current may be deter-
mined by the following equation:
The master’s high current-limit threshold must be set
high enough to support the maximum load current,
even when the master’s current-limit threshold is at its
minimum tolerance value, as described in the master
controller’s data sheet. Most Quick-PWM controllers
that may be chosen as the master controller use the
low-side MOSFET’s on-resistance to sense the inductor
current. In these applications, the worst-case maximum
value for R
R
the master’s current-limit threshold. A good general rule
is to allow 0.5% additional resistance for each °C of
temperature rise. Set the master current-limit threshold
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
Figure 5. Setting the Adjustable Current Limits
20
LIMIT(VALLEY)
DS(ON)
I
LIMIT VALLEY
______________________________________________________________________________________
over temperature must be used to determine
(
∆I
MAX1887
MAX1897
DS(ON)
INDUCTOR
CONTROLLER
) occurs at I
)
SLAVE
≥
LIMIT
ILIM



plus some margin for the rise in
I
LOAD MAX
=
V
η
OUT IN
(
R
D
LOAD(MAX)
R
LIMIT
R
V f
C
C
(
IN SW
)
V
LIMIT



−
−



V
L
∆
OUT
C
I
INDUCTOR
REF
R
R
A
B
divided by the
)
2
MASTER
CONTROLLER
REF
ILIM



MAX1718
to support the maximum load current for the maximum
R
where V
ically 1/10th the voltage seen at the master’s ILIM input
(V
troller’s data sheet). Connect a resistive voltage-divider
from the master controller’s internal reference to GND,
with the master’s ILIM input connected to the center tap
(Figure 5). Use 1% tolerance resistors in the divider with
10µA to 20µA DC bias current to prevent significant
errors due to the ILIM pin’s input current:
Configure the slave controller so its LIMIT output begins
to roll off after the master current-limit threshold occurs:
where V
cisely one-tenth the voltage seen at the slave’s ILIM
input (V
resistive voltage-divider from the master controller’s
internal reference to GND, with the slave’s ILIM input
connected to the center tap (Figure 5). The external
adjustment range of 400mV to 1.5V corresponds to a
DS(ON)
ITHM
V
V
V
ITHM HIGH
ITHM HIGH
ITHS
V
V
ITHM(HIGH)
ITHS
(
(
ITHM
ITHS
= 0.1 x V
ITHS
=
and minimum current-limit tolerance value:
R
V
10
ILIM MASTER
A
1
, the slave’s current-limit threshold, is pre-
=
, the master’s current-limit threshold, is typ-
≥



= 0.1
)
)
20
(
R




R
=
=



C
µ
CM
10
10
V
V
R
1
1
A
REF MASTER
ILIM MASTER
+
D
LIM(MASTER)
≥ (I
R









✕
R
R
D
R
(
(
V
V
LIMIT(VALLEY)
A
A
)
DS ON MAX



ITHM HIGH
R
ILIM(SLAVE)
R
V
≤
+
+
B
REF
(
B
(
R
R
R
//
B
B
B
(
R
)(



≤
//
)
)
LIMIT
V
R



V
REF
ILIM MASTER
, see the master con-
LIMIT
−
)
1
)




). Connect a second
10
R
)R
+
(
B
∆
)
µ
DS(ON)(MAX)



I
A
INDUCTOR
V
REF
)


