Owner manual

MAX1864/MAX1865
xDSL/Cable Modem Triple/Quintuple Output
Power Supplies
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Do not use output capacitors with more than 200mΩ of
ESR. Typically, more output capacitance provides the
best solution, since this also reduces the output voltage
drop immediately after a load transient.
Linear Regulator Output Capacitors
Connect at least a 1µF capacitor between the linear
regulator’s output and ground, as close to the
MAX1864/MAX1865 and external pass transistors as
possible. Depending on the selected pass transistor,
larger capacitor values may be required for stability
(see Stability Requirements). Furthermore, the output
capacitor’s ESR affects stability, providing a zero that
may be necessary to cancel the second pole. Use out-
put capacitors with an ESR less than 200mΩ to ensure
stability and optimum transient response.
Once the minimum capacitor value for stability is deter-
mined, verify that the linear regulator’s output does not
contain excessive noise. Although adequate for stabili-
ty, small capacitor values may provide too much band-
width, making the linear regulator sensitive to noise.
Larger capacitor values reduce the bandwidth, thereby
reducing the regulator’s noise sensitivity.
If noise on the ground reference causes the design to
be marginally stable for the negative linear regulator,
bypass the negative output back to its reference volt-
age (V
REF
, Figure 7). This technique reduces the differ-
ential noise on the output.
Base-Drive Noise Reduction
The high-impedance base driver is susceptible to sys-
tem noise, especially when the linear regulator is lightly
loaded. Capacitively coupled switching noise or induc-
tively coupled EMI onto the base drive causes fluctua-
tions in the base current, which appear as noise on the
linear regulator’s output. Keep the base-drive traces
away from the step-down converter and as short as
possible to minimize noise coupling. Resistors in series
with the gate drivers (DH and DL) reduce the LX
switching noise generated by the step-down converter
(Figure 5). Additionally, a bypass capacitor may be
placed across the base-to-emitter resistor (Figure 7).
This bypass capacitor, in addition to the transistor’s
input capacitance, could bring in a second pole that
will destabilize the linear regulator (see Stability
Requirements). Therefore, the stability requirements
determine the maximum base-to-emitter capacitance:
where C
IN(Q)
is the transistor’s input capacitance, and
f
POLE(CBE)
is the second pole required for stability.
Transformer Selection
In systems where the step-down controller’s output is
not the highest voltage, a transformer may be used to
provide additional postregulated, high-voltage outputs.
The transformer generates unregulated, high-voltage
supplies that power the positive and negative linear
regulators. These unregulated supply voltages must be
high enough to keep the pass transistors from saturat-
ing. For positive output voltages, connect the trans-
former as shown in figure 6 where the minimum turns
ratio (N) is determined by:
where V
SAT
is the pass transistor’s saturation voltage
under full load. For negative output voltages (MAX1865
N
VVV
V
POS
LDO POS SAT DIODE
OUT
≥
++






()
-1
C
RI Vh
RVh
C
BE
POLE CBE
BE LOAD T FE
BE T FE
IN Q
≤
ƒ
+






1
2π
()
()
-
ƒ≈
POLE FB
OUT ESR
CR
()
1
2π
C
BYP
V
NEG
V
SUP
Q
PASS
C
LDO
R
BE
C
BE
R1
R2
B_
a) POSITIVE OUTPUT VOLTAGE
b) NEGATIVE OUTPUT VOLTAGE (MAX1865 ONLY)
FB_
MAX1864
MAX1865
V
POS
C
BYP
V
REF
V
SUP
Q
PASS
C
NEG
R
BE
C
BE
R4
R3
BF5
B5
MAX1865
Figure 7. Base-Drive Noise Reduction