MAX1887EEE Maxim Integrated Products, MAX1887EEE Datasheet - Page 21

• Manufacturer Part Number: MAX1887EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1887EEE

• Number Of Outputs: 1
• Mounting Style: SMD/SMT
• Package / Case: QSOP-16
• Switching Frequency: 550 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost

current-limit threshold of 40mV to 150mV. Use 1% toler-
ance resistors in the divider with 10µA to 20µA DC bias
current to prevent significant errors due to the ILIM
pin’s input current. Reducing the current-limit threshold
voltage lowers the sense resistor’s power dissipation,
but this also increases the relative measurement error:
Now, set the current-limit adjustment ratio (A
V
minimum on-resistance ratio (A
R
Increasing A
accuracy but also increases the current limit’s noise
sensitivity. Therefore, R
following equation:
Finally, verify that the total load on the master’s refer-
ence does not exceed 50µA:
Current Limit Design Example
For the typical application circuit shown in Figure 1: V
= 12V, V
= 50A, L = 0.6µH, R
= 3mΩ
1) Determine the peak-to-peak inductor current and
I
BIAS TOTAL
ITHM(HIGH)
DS(ON)(MIN)
the valley current limit:
(
I
∆I
LIMIT VALLEY
R
INDUCTOR
LIMIT
OUT
R
(
V
)
/V
1
ILIM SLAVE
C
=
):
= 1.3V, f
+
ADJ
ITHM(LOW)
≤
=



20



R
R
(
R




R



µ
DS ON MAX
A
A
______________________________________________________________________________________
LIMIT
improves the master’s current-limit
=
V
A
)
(
V
A
+
REF MASTER
//
R
(
=
12
DS(ON)(MAX)
ILIM SLAVE
ADJ
R
(
A
R



1 3
SW
B
V
V x
)
.
50
(
//
B
LIMIT
)(
REF
(

 ≥

≤
) greater than the maximum to
2
R R
V x
//
≥
A
300
R
B
= 300kHz,
R
A
D



R
)
R
(
LIMIT
12
ROS
)
−
DS ON MAX
may be selected using the
DS ON MIN
≤
kHz x
DS ON MIN
−



)
V
V
R
2
)
(
1
(
(
ILIM SLAVE



DS ON MIN
−
)
x
RDS
= 6mΩ, R
1 3

 +

−
6 4
10
0 6
)(
.
1
(
)(
)(
.
(
.




V
R



µ
Quick-PWM Slave Controllers for
η
A
µ
Multiphase, Step-Down Supplies
R
A
= R
)
D
)(
H

 =

= 2, I
C
V
)
)
)
REF
=
+
DS(ON)(MAX)
21 8
)
6 4
R
DS(ON)(MIN )
)
.
D
.
LOAD(MAX)
A

 ≤ 50µ

A
ADJ
A
IN
=
/
2) Determine the master’s current-limit threshold from
3) Determine the slave’s current-limit threshold:
4) Determine R
5) Finally, verify that that the total bias currents do not
R
the valley current limit and low-side MOSFETs’ max-
imum on-resistance over temperature:
Now select the resistive-divider values (R
in Figure 5) to set the appropriate voltage at the
master’s ILIM input:
Selecting R
value for R
Select the resistive-divider values (R
Figure 5) to set the appropriate voltage at the
slave’s ILIM input:
Selecting R
value for R
exceed the 50µA maximum load of the master’s ref-
erence:
B
R
D
R
=
V
ITHS
LIMIT
I






=
BIAS TOTAL
V
10 130
R
30 1
ITH(MASTER)



R
A
10 42
x
C
.
20
≥
(
=
≤
k
20
=
1 5
x
µ
A
A
Ω



.
D
(
B



2
A
10 130
:
:
µ
LIMIT
53 6
mV
+
10 42
V
m x
A
mV
= 30.1kΩ ±1% provides the following
= 100kΩ ±1% provides the following
113
x
.
Ω
)
x
2



2
k
=
V



6
to
(Figure 5) from the above equation:
V
Ω
k



m



to
≥ 21.8A
Ω
mV
mV
130



//
54
Ω
10 130
6




 =

100
m
10 42
k
−
−
mV
−
Ω
x
10
Ω
10
36
1
3
x
1
k



m
+



µ
Ω
µ
µ
x
✕
(
+
x
A
Ω
100
)
A
A
100
mV
mV
30 1
x m
6mΩ = 130mV
6 4
.
3
2
. Ω
k
k
V

 =


 =

A
Ω
k
Ω
Ω

 ≈

//
≈
21
65
≈
≈
34 8
54
113
42
k to
35
k to
.
Ω
k
Ω
C
mV
Ω
k
k
k
Ω
Ω
and R
Ω
A
42
130
)

 +

and R
k
Ω
k
Ω
D
21
in
B