MAX1864TEEE+ Maxim Integrated Products, MAX1864TEEE+ Datasheet - Page 18

• Manufacturer Part Number: MAX1864TEEE+
• Description: IC PWR SUPPLY CONTROLLER 16QSOP
• Manufacturer: Maxim Integrated Products

Specifications of MAX1864TEEE+

• Applications: Power Supply Controller
• Voltage - Input: 4.5 ~ 28 V
• Current - Supply: 1mA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-QSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
where A
typ), A
typ), and V
the feedback resistive-divider (internal or external).
Since the output voltage is a function of the load cur-
rent and load resistance, the total DC loop gain
(A
The compensation capacitor (C
inant pole. Due to the current-mode control scheme,
the output capacitor also creates a pole in the system
that is a function of the load resistance. As the load
resistance increases, the frequency of the output
capacitor’s pole decreases. However, the DC loop gain
increases with larger load resistance, so the unity gain
bandwidth remains fixed. Additionally, the compensa-
tion resistor and the output capacitor’s ESR both gener-
ate zeros. Therefore, to achieve stable operation, use
the following procedure to properly compensate the
system:
1) First, select the desired crossover frequency. The
2) Next, determine the pole set by the output capacitor
3) Determine the compensation resistor required to set
18
A
V(DC)
V DC
crossover frequency must be less than both 1/5th
the switching frequency and 1/3rd the zero frequen-
cy set by the output capacitor’s ESR:
and the load resistor:
the desired crossover frequency:
(
ƒ
______________________________________________________________________________________
POLE OUT
VEA
)
) is approximately:
≈
VCS
I
PEAK
I
I
(
LOAD
PEAK
R
is the DC gain of the error amplifier (2000
OUT(NOMINAL)
COMP
ƒ ≤
is the current-sense amplifier’s gain (4.9
c
)
=
=
≈
≈
V
6
2
OUT NOMINAL DS ON
V
V
=
π
π
OUT NOMINAL DS ON
OUT NOMINAL DS ON
C
C
g A
400
OUT LOAD
(
OUT ESR
m V DC POLE OUT
V
(
(
1
V
OUT REF VEA
1
REF LOAD VEA
×
R
R
2000
(
is the output voltage set by
V
REF LOAD
V
R
)
ƒ
COMP
)
R
R
× ƒ
and
)
)
R
A
R
=
c
2
A
(
I
π
(
) creates the dom-
LOAD MAX
(
ƒ
(
C
SW
5
)
OUT OUT
A
)
)
)
A
VCS
(
VCS
V
)
4) Finally, select the compensation capacitor:
A signal diode, such as the 1N4148, works well in most
applications. If the input voltage goes below 6V, use a
small 20mA Schottky diode for slightly improved effi-
ciency and dropout characteristics. Do not use large
power diodes, such as the 1N5817 or 1N4001, since
high junction capacitance can charge up VL to exces-
sive voltages.
The MAX1864/MAX1865s’ positive linear regulator out-
put voltages are set by connecting a voltage-divider
from the output to FB_ to GND (Figure 6). Select R4 in
the 5kΩ to 50kΩ range. Calculate R3 with the following
equation:
where V
30V.
The MAX1865’s negative output voltage is set by con-
necting a voltage-divider from the output to FB5 to a
positive voltage reference (Figure 6). Select R6 in the
5kΩ to 50kΩ range. Calculate R5 with the following
equation:
where V
V
If the negative regulator is used, the OUT pin must be
connected to a voltage supply between 2V and 5V that
can source at least 25mA. Typically, the OUT pin is
connected to the step-down converter’s output.
However, if the step-down converter’s output voltage is
set higher than 5V, OUT may be connected to one of
the positive linear regulators with an output voltage
between 2V and 5V.
OUT
where the error amplifier’s transconductance (g
100µS (see Electrical Characteristics).
may be set between 0 and -20V.
Negative Output Voltage Selection (MAX1865)
FB
REF
= 1.24V, and V
C
is the positive reference voltage used, and
COMP
R
3
R
=
Linear Regulator Controllers
≤
5
Positive Output Voltage Selection
R
2π
=
4
R
⎡
⎢
⎢
⎣
R
⎛
⎜
⎝
6
COMP POLE OUT
V
⎛
⎜
⎝
OUT
V
OUT
V
FB
V
OUT
REF
may range from 1.24V to
ƒ
1
⎞
⎟
⎠
-
⎞
⎟
⎠
1
Boost-Supply Diode
⎤
⎥
⎥
⎦
(
)
m
) is