Rainbow Electronics MAX1865 User Manual
Page 17
MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
______________________________________________________________________________________
17
Output Capacitor
The key selection parameters for the output capacitor
are the actual capacitance value, the equivalent series
resistance (ESR), and voltage-rating requirements,
which affect the overall stability, output ripple voltage,
and transient response.
The output ripple has two components: variations in the
charge stored in the output capacitor, and the voltage
drop across the capacitor’s ESR caused by the current
into and out of the capacitor:
The output voltage ripple as a consequence of the ESR
and output capacitance is:
where I
P-P
is the peak-to-peak inductor current (see
Inductor Selection). These equations are suitable for
initial capacitor selection, but final values should be set
by testing a prototype or evaluation circuit. As a gener-
al rule, a smaller ripple current results in less output rip-
ple. Since the inductor ripple current is a factor of the
inductor value and input voltage, the output voltage rip-
ple decreases with larger inductance but increases
with lower input voltages.
With low-cost aluminum electrolytic capacitors, the
ESR-induced ripple can be larger than that caused by
the current into and out of the capacitor. Consequently,
high-quality low-ESR aluminum-electrolytic, tantalum,
polymer, or ceramic filter capacitors are required to
minimize output ripple. Best results at reasonable cost
are typically achieved with an aluminum-electrolytic
capacitor in the 470µF range, in parallel with a 0.1µF
ceramic capacitor.
Since the MAX1864/MAX1865 use a current-mode con-
trol scheme, the output capacitor forms a pole that
affects circuit stability (see Compensation Design).
Furthermore, the output capacitor’s ESR also forms a
zero.
The MAX1864/MAX1865s’ response to a load transient
depends on the selected output capacitor. After a load
transient, the output instantly changes by ESR
✕
∆I
LOAD
. Before the controller can respond, the output
will sag further, depending on the inductor and output
capacitor values.
After a short period of time (see Typical Operating
Characteristics), the controller responds by regulating
the output voltage back to its nominal state. For appli-
cations that have strict transient requirements, low-ESR
high-capacitance electrolytic capacitors are recom-
mended to minimize the transient voltage swing.
Do not exceed the capacitor’s voltage or ripple-current
ratings.
Compensation Design
The MAX1864/MAX1865 controllers use an internal
transconductance error amplifier whose output com-
pensates the control loop. Connect a series resistor
and capacitor between COMP and GND to form a pole-
zero pair. The external inductor, high-side MOSFET,
output capacitor, compensation resistor, and compen-
sation capacitor determine the loop stability. The induc-
tor and output capacitor are chosen based on
performance, size, and cost. Additionally, the compen-
sation resistor and capacitor are selected to optimize
control-loop stability. The component values shown in
the standard application circuits (Figures 1 and 6) yield
stable operation over a broad range of input-to-output
voltages.
The controller uses a current-mode control scheme that
regulates the output voltage by forcing the required
current through the external inductor, so the
MAX1864/MAX1865 use the voltage across the high-
side MOSFET’s R
DS(ON)
to sense the inductor current.
Using the current-sense amplifier’s output signal and
the amplified feedback voltage, the control loop deter-
mines the peak inductor current by:
V
I
ESR
V
I
C
I
V
V
L
V
V
RIPPLE ESR
p p
RIPPLE C
p p
OUT SW
p p
IN
OUT
SW
OUT
IN
(
)
( )
=
=
ƒ
=
ƒ
-
-
-
-
2
V
V
V
RIPPLE
RIPPLE ESR
RIPPLE C
=
+
(
)
( )
BST
N
H
R
GATE
(OPTIONAL)
N
L
R
GATE
(OPTIONAL)
C
BST
DH
LX
L
TO VL
DH
GND
MAX1864
MAX1865
Figure 5. Reducing the Switching EMI