Rainbow Electronics MAX8884Z - Manual (Page 11)

MAX8884Y/MAX8884Z
700mA DC-DC Step-Down Converters
with Dual 300mA LDO in 2mm x 2mm CSP
______________________________________________________________________________________ 11
The inductor’s DC current rating only needs to match the
maximum load of the application because the
MAX8884Y/MAX8884Z 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 DC-DC step-down converter, the output capacitor
C
BUCK is required to keep the output voltage ripple small
and ensure regulation loop stability. C BUCK 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 out-
put capacitance can be very low. A 2.2μF ceramic capaci-
tor 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
lighter than 10mA, it is sufficient to use a 0.1μF ceramic
capacitor for stable operation over the full temperature
range. For loads up to 200mA, an output capacitor of
1μF is sufficient for stable operation over the entire tem-
perature range. Operating the LDO at maximum rated
current the LDO1 and LDO2 requires a 2.2μF ceramic
capacitor. Using larger output capacitors reduces out-
put noise and improves load-transient response, stabili-
ty, and power-supply rejection.
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 4.7μF or more
to ensure stability at temperatures below -10°C. With X7R
or X5R dielectrics, 2.2μF is sufficient at all operating tem-
peratures. These regulators are optimized for ceramic
capacitors. Tantalum capacitors are not recommended.
Input Capacitor Selection
The input capacitor (C IN1 ) of the DC-DC step-down
converter reduces the current peaks drawn from the
battery or input power source and reduces switching
noise in the MAX8884Y/MAX8884Z. The impedance of
C
IN1
at the switching frequency should be kept very
low. Ceramic capacitors with X5R or X7R dielectric are
highly recommended due to their small size, low ESR,
and small temperature coefficients. A 2.2μF ceramic
capacitor is recommended for most applications. For
optimum noise immunity and low input ripple, the input
capacitor value can be increased.
For the LDOs, use an input capacitance equal to the
value of the sum of the output capacitance of LDO1 and
LDO2. Larger input capacitor values and lower ESR pro-
vide better noise rejection and line transient response.
Note that some ceramic dielectrics exhibit large capaci-
tance and ESR variation with temperature. With dielectrics
such as Z5U and Y5V, it may be necessary to use two
times the sum of the output capacitor value of LDO1 and
LDO2 (or larger) to ensure stability at temperatures below
-10°C. With X7R or X5R dielectrics, a capacitance equal
to the sum is sufficient at all operating temperatures.
Reference Noise
Bypass Capacitor Selection
The REFBP capacitor reduces the output noise of LDO1
and LDO2. A value of 0.033μF is sufficient for most appli-
cations. This value can be increased up to 0.150μF with
some effect on the soft-start time of the LDOs. See the
Typical Operating Characteristics for more information.
Do not use values greater than 0.150μF as this degrades
the performance of the internal reference voltage and has
a corresponding impact on all output voltages.
Ceramic capacitors with X5R or X7R dielectric are high-
ly recommended due to their small size, low ESR, and
small temperature coefficients. Note that some ceramic
dielectrics exhibit large capacitance and ESR variation
with temperature. With dielectrics such as Z5U and
Y5V, it may be necessary to use two times the recom-
mended value to achieve desired output noise perfor-
mance at temperatures below -10°C. Tantalum
capacitors are not recommended.
Thermal Considerations
In most applications, the MAX8884Y/MAX8884Z do not
dissipate much heat due to their high efficiency. But in
applications where the MAX8884Y/MAX8884Z run at high
ambient temperature with heavy loads, the heat dissipat-
ed may exceed the maximum junction temperature of the
part. If the junction temperature reaches approximately
+160°C, all power switches are turned off and LX and FB
become high impedance, and LDO1 and LDO2 are
pulled down to ground through an internal 100
Ω resistor.
The MAX8884Y/MAX8884Z 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, P
DISS , is:
where η
BUCK is the efficiency of the DC-DC step-down
converter, and P BUCK is the output power of the DC-DC
step-down converter.
P P
BUCK
I V V I V V
DISS BUCK LDO IN LDO LDO IN LDO
= −
⎛
⎝⎜ ⎞
⎠⎟
+ − + −
1
1
1 2 1 2 2 2 η
( ) ( )