Rainbow Electronics MAX8805Z - Manual (Page 14)

MAX8805Y/MAX8805Z
600mA PWM Step-Down Converters in
2mm x 2mm WLP for WCDMA PA Power
14 ______________________________________________________________________________________
Step-Down Converter Soft-Start
The MAX8805Y/MAX8805Z PA step-down converter has
internal soft-start circuitry that limits inrush current at
startup, reducing transients on the input source. Soft-
start is particularly useful for supplies with high output
impedance such as Li+ and alkaline cells. See the Soft-
Start Waveforms in the
Typical Operating Characteristics.
Analog REFIN Control
The MAX8805Y/MAX8805Z PA step-down converter uses
REFIN to set the output voltage. The output voltage is reg-
ulated at twice the voltage applied at REFIN minus the
load regulation. This allows the converter to operate in
applications where dynamic voltage control is required.
Thermal Shutdown
Thermal shutdown limits total power dissipation in the
MAX8805Y/MAX8805Z. If the junction temperature
exceeds +160°C, thermal-shutdown circuitry turns off
the IC, allowing it to cool. The IC turns on and begins
soft-start after the junction temperature cools by 20°C.
This results in a pulsed output during continuous ther-
mal-overload conditions.
Applications Information
Output Voltages
The MAX8805Y/MAX8805Z PA step-down converters
set the PA_ output voltage to twice the voltage applied
to REFIN.
LDO1 and LDO2 output voltages are determined by the
part number suffix, as shown in Table 1.
LDO Dropout Voltage
The regulator’s minimum input/output differential (or
dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines
the useful end-of-life battery voltage. Because the
MAX8805Y/MAX8805Z LDOs use a p-channel MOSFET
pass transistor, their dropout voltages are a function of
drain-to-source on-resistance (R
DS (ON)) multiplied by the
load current (see the Typical Operating Characteristics ).
Inductor Selection
The MAX8805Y operates with a switching frequency of
2MHz and utilizes a 2.2µH inductor. The MAX8805Z
operates with a switching frequency of 4MHz and uti-
lizes a 1µH inductor. The higher switching frequency of
the MAX8805Z allows the use of physically smaller
inductors at the cost of slightly lower efficiency. The
lower switching frequency of the MAX8805Y results in
greater efficiency at the cost of a physically larger
inductor. See the
Typical Operating Characteristics for
efficiency graphs for both the MAX8805Y and
MAX8805Z.
The inductor’s DC current rating only needs to match the
maximum load of the application because the
MAX8805Y/MAX8805Z feature zero current overshoot
during startup and load transients. For optimum transient
response and high efficiency, choose an inductor with
DC series resistance in the 50m
Ω to 150mΩ range. See
Table 2 for suggested inductors and manufacturers.
Output Capacitor Selection
For the PA step-down converter, the output capacitor
(C
PA ) is required to keep the output voltage ripple small
and ensure regulation loop stability. C PA must have low
impedance at the switching frequency. Ceramic capaci-
tors with X5R or X7R dielectric are highly recommended
due to their small size, low ESR, and small temperature
coefficients. Due to the unique feedback network, the
output capacitance can be very low. A 2.2µF capacitor
is recommended for most applications. For optimum
load-transient performance and very low output ripple,
the output capacitor value can be increased.
For LDO1 and LDO2, the minimum output capacitance
required is dependent on the load currents. For loads
less than 10mA, it is sufficient to use a 0.1µF capacitor
for stable operation over the full temperature range.
With rated maximum load currents, a minimum of 1µF is
recommended. Reduce output noise and improve load-
transient response, stability, and power-supply rejec-
tion by using larger output capacitors.
Note that some ceramic dielectrics exhibit large capaci-
tance and ESR variation with temperature. With dielectrics
such as Z5U and Y5V, it is necessary to use 2.2µF or larg-
er to ensure stability at temperatures below -10°C. With
X7R or X5R dielectrics, 1µF is sufficient at all operating
temperatures. These regulators are optimized for ceramic
capacitors. Tantalum capacitors are not recommended.
Table 1. LDO1 and LDO2 Output Voltage
Selection
Note: Contact the factory for other output-voltage options.
PART
FREQUENCY
(MHz)
LDO1
(V)
LDO2
(V)
MAX8805YEWEAA+T 2 1.80 1.80
MAX8805YEWEAE+T 2 1.80 2.85
MAX8805YEWEEE+T 2 2.85 2.85
MAX8805ZEWEAA+T 4 1.80 1.80
MAX8805ZEWEAE+T 4 1.80 2.85
MAX8805ZEWEEE+T 4 2.85 2.85