Rainbow Electronics MAX1865 User Manual
Page 18
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
18
______________________________________________________________________________________
where A
VCS
is the current-sense amplifier’s gain (4.9
typ), A
VEA
is the DC gain of the error amplifier (2000
typ), and V
OUT(NOMINAL)
is the output voltage set by
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
V(DC)
) is approximately:
The compensation capacitor (C
COMP
) creates the dom-
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
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:
2) Next, determine the pole set by the output capacitor
and the load resistor:
3) Determine the compensation resistor required to set
the desired crossover frequency:
where the error amplifier’s transconductance (g
m
) is
100µS (see Electrical Characteristics).
4) Finally, select the compensation capacitor:
Boost-Supply Diode
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.
Linear Regulator Controllers
Positive Output Voltage Selection
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 5k
Ω to 50kΩ range. Calculate R3 with the following
equation:
where V
FB
= 1.24V, and V
OUT
may range from 1.24V to
30V.
Negative Output Voltage Selection (MAX1865)
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
5k
Ω to 50kΩ range. Calculate R5 with the following
equation:
where V
REF
is the positive reference voltage used, and
V
OUT
may be set between 0 and -20V.
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.
R
R
V
V
OUT
REF
5
6
=
R
R
V
V
OUT
FB
3
4
1
=
-
C
R
COMP
COMP POLE OUT
≤
ƒ
1
2
π
(
)
R
g A
COMP
c
m V DC
POLE OUT
=
× ƒ
ƒ
2000
(
)
(
)
ƒ
=
=
POLE OUT
OUT LOAD
LOAD MAX
OUT OUT
C
R
I
C
V
(
)
(
)
1
2
2
π
π
ƒ ≤
ƒ
c
OUT ESR
SW
C
R
and
1
6
5
π
V
R
V
R
REF LOAD
OUT NOMINAL
DS ON
(
)
(
)
≈
×
400
A
I
I
V
R
A
V
R
A
V DC
PEAK
LOAD
REF LOAD VEA
OUT NOMINAL
DS ON
VCS
(
)
(
)
(
)
≈
≈
I
V
V
A
V
R
A
PEAK
OUT REF VEA
OUT NOMINAL
DS ON
VCS
=
(
)
(
)