MAX8896
Dual PWM Step-Down Converter in a 2mm x
2mm Package for WCDMA PA and RF Power
______________________________________________________________________________________ 13
The OUT2 step-down DC-DC converter operates with
100% duty cycle when the supply voltage approaches
the output voltage. This allows this converter to main-
tain regulation until the input voltage falls below the
desired output voltage plus the dropout voltage specifi-
cation of the converter. During 100% duty cycle opera-
tion, the high-side p-channel MOSFET turns on
constantly, connecting the input to the output through
the inductor. The dropout voltage (V
DO
) is calculated
as follows:
V
DO
= I
LOAD
x (R
P
+ R
L
)
where:
R
P
= internal p-channel MOSFET switch on-resistance
(see
Electrical Characteristics
)
R
L
= external inductor DC resistance
LDO
The LDO provides 200mA at 2.8V and is designed for
low noise (16µV
RMS
, typ) and high PSRR (65dB, typ).
The LDO is powered from OUT2 (3.1V) and is enabled
or disabled at the same time as OUT2 using RFEN1 or
RFEN2.
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 LDO
uses a p-channel MOSFET pass transistor, the dropout
voltage is drain-to-source on-resistance (R
DS(ON)
) mul-
tiplied by the load current (see the
Typical Operating
Characteristics
).
Shutdown Mode
Connect PAEN to GND or logic-low to place OUT1 in
shutdown mode. In shutdown, the control circuitry,
internal switching MOSFET, and synchronous rectifier
turn off and LX1 becomes high impedance. Connect
PAEN to IN1, V
CC
, or logic-high for normal operation.
Either RFEN1 or RFEN2 enable OUT2 and the LDO.
Connect RFEN1 and RFEN2 to GND or logic-low to
place OUT2 and the LDO in shutdown mode. In shut-
down, the control circuitry, internal switching MOSFET,
and synchronous rectifier turn off and LX2 and the LDO
output become high impedance. Connect RFEN1 or
RFEN2 to IN2, V
CC
, or logic-high for normal operation.
When PAEN, RFEN1, and RFEN2 are all logic-low, the
MAX8896 enter a very low power state, where the input
current drops to 0.1µA (typ).
Thermal-Overload Protection
Thermal-overload protection limits total power dissipation
in the MAX8896. If the junction temperature exceeds
+160°C, the MAX8896 turn off, allowing the IC to cool.
The IC turns on and begins soft-start after the junction
temperature cools by 20°C. This results in a pulsed out-
put during continuous thermal-overload conditions.
Applications Information
Inductor Selection
OUT1 operates with a switching frequency of 2MHz and
utilizes a 2.2µH to 4.7µH inductor. OUT2 operates with a
switching frequency of 2MHz and utilizes a 2.2µH induc-
tor. This operating frequency allows the use of physically
small inductors while maintaining high efficiency.
The OUT1 inductor’s DC current rating only needs to
match the maximum load of the application because
OUT1 features zero current overshoot during startup and
load transients. For optimum transient response and high
efficiency, choose an inductor with DC series resistance
in the 50mΩ to 150mΩ range. See Table 1 for suggested
inductors and manufacturers.
Using a larger inductance value reduces the ripple cur-
rent, therefore providing higher efficiency at light load.
Output Capacitor Selection
For OUT1 and OUT2, the output capacitor keeps the
output voltage ripple small and ensures regulation loop
stability. C
OUT
must have low impedance at the switch-
ing frequency. Ceramic capacitors with X5R or X7R
temperature characteristics are highly recommended
due to their small size, low ESR, and small temperature
coefficients. A 4.7µF capacitor is recommended for
C
OUT1
and 2.2µF is recommended for C
OUT2
. For opti-
mum load-transient performance and very low output
ripple, the output capacitor value can be increased.
For the LDO, the minimum output capacitance required
is dependent on the load currents. For loads lighter
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 rec-
ommended. Larger value output capacitors further
reduce output noise and improve load-transient
response, stability, and power-supply rejection.
Note that some ceramic dielectrics exhibit large capac-
itance and ESR variation with temperature and DC bias.
Ceramic capacitors with Z5U or Y5V temperature char-
acteristics should be avoided. These regulators are
optimized for ceramic capacitors. Tantalum capacitors
are not recommended.
