Rainbow Electronics MAX8508 - Manual (Page 9)

MAX8506/MAX8507/MAX8508
PWM Step-Down DC-DC Converters with 75m Ω
Bypass FET for WCDMA and cdmaOne Handsets
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capacitor ripple. This sets the device speed for transient
response and allows the use of small ceramic output
capacitors because the phase-shifted capacitor ripple
does not disturb the current-regulation loop. The resistor
sets the proportional gain of the output error voltage by
a factor of g
m x R C . Increasing this resistor also increas-
es the sensitivity of the control loop to output ripple.
The resistor and capacitor set a compensation zero that
defines the system’s transient response. The load cre-
ates a dynamic pole, shifting in frequency with changes
in load. As the load decreases, the pole frequency
shifts to the left. System stability requires that the com-
pensation zero must be placed to ensure adequate
phase margin (at least 30° at unity gain). With a 4.7µF
output capacitor, the recommended C
C and R C for the
MAX8506 are 1500pF and 10kΩ, respectively. This pro-
vides adequate phase margin over the entire output
voltage and load range and optimizes the output-
voltage settling time for REFIN dynamic control. See the
Typical Application Circuits for recommended C
C
and
R C values for the MAX8507 and MAX8508.
Inductor Selection
A 4µH to 6µH inductor is recommended for most appli-
cations. For best efficiency, the inductor’s DC resistance
should be <400mΩ. Saturation current (I
SAT ) should be
greater than the maximum DC load at the PA’s supply
plus half the inductor current ripple. Two-step V
CC
applications typically require very small inductors with
I
SAT
in the 200mA to 300mA region. See Tables 1 and 2
for recommended inductors and suppliers.
PC Board Layout and Routing
High switching frequencies and large peak currents
make PC board layout a very important part of design.
Good design minimizes EMI, noise on the feedback
paths, and voltage gradients in the ground plane, all of
which can result in instability or regulation errors.
Connect the inductor, input filter capacitor, and output fil-
ter capacitor as close together as possible and keep their
traces short, direct, and wide. The external voltage- feed-
back network should be very close to the FB pin, within
0.2in (5mm). Keep noisy traces, such as those from the
LX pin, away from the voltage-feedback network. Position
the bypass capacitors as close as possible to their
respective supply and ground pins to minimize noise cou-
pling. For optimum performance, place input and output
capacitors as close to the device as possible. Connect
GND directly under the IC to the exposed paddle. Refer
to the MAX8506 evaluation kit for an example PC board
layout and routing scheme.
SUPPLIER PHONE WEBSITE
Murata 770-436-1300 www.murata.com
Sumida 847-956-0666 www.sumida.com
Taiyo Yuden 408-573-4150 www.t-yuden.com
TOKO 847-297-0070 www.tokoam.com
Table 2. Component Suppliers
Table 1. Suggested Inductors
SUPPLIER
PART
NUMBER
INDUCTANCE
(µH)
ESR (m
Ω
)
SATURATION
CURRENT (A)
DIMENSIONS (mm)
Murata LQH32C-53 4.7 150 0.650 2.5 x 3.2 x 1.7
Sumida CDRH2D11 4.7 135 0.500 3.2 x 3.2 x 1.2
Taiyo Yuden LBLQ2016 4.7 250 0.210 1.6 x 2.0 x 1.6
TOKO D312C 4.7 200 0.790 3.6 x 3.6 x 1.2