Rainbow Electronics MAX8805Z - Manual (Page 16)

MAX8805Y/MAX8805Z
600mA PWM Step-Down Converters in
2mm x 2mm WLP for WCDMA PA Power
16 ______________________________________________________________________________________
Thermal Considerations
In most applications, the MAX8805Y/MAX8805Z do not
dissipate much heat due to their high efficiency.
However, in applications where the MAX8805Y/
MAX8805Z run at high ambient temperature with heavy
loads, the heat dissipated may exceed the maximum
junction temperature of the IC. If the junction tempera-
ture reaches approximately +160°C, all power switches
are turned off and LX and PA_ become high imped-
ance, and LDO1 and LDO2 are pulled down to ground
through an internal 1k
Ω pulldown resistor.
The MAX8805Y/MAX8805Z maximum power dissipation
depends on the thermal resistance of the IC package
and circuit board, the temperature difference between
the die junction and ambient air, and the rate of airflow.
The power dissipated in the device is:
P DISS = P PA x (1/
η
PA - 1) + I LDO1 x (V IN2 - V LDO1 ) +
I LDO2 x (V IN2 -V LDO2 )
where
η
PA is the efficiency of the PA step-down con-
verter and P PA is the output power of the PA step-down
converter.
The maximum allowed power dissipation is:
P
MAX
= (T
JMAX
- T
A
) / θ
JA
where (T JMAX - T A ) is the temperature difference
between the MAX8805Y/MAX8805Z die junction and
the surrounding air;
θ
JA is the thermal resistance of the
junction through the PCB, copper traces, and other
materials to the surrounding air.
PCB Layout
High switching frequencies and relatively large peak
currents make the PCB layout a very important part of
design. Good design minimizes excessive EMI on the
feedback paths and voltage gradients in the ground
plane, resulting in a stable and well-regulated output.
Connect C
IN1
close to IN1A/IN1B and PGND. Connect
the inductor and output capacitor as close as possible
to the IC and keep their traces short, direct, and wide.
Keep noisy traces, such as the LX node, as short as
possible. Figure 3 illustrates an example PCB layout
and routing scheme.