Rainbow Electronics MAX8884Z - Manual (Page 10)

MAX8884Y/MAX8884Z
700mA DC-DC Step-Down Converters
with Dual 300mA LDO in 2mm x 2mm CSP
10 ______________________________________________________________________________________
the off period, the low-side synchronous rectifier turns
on and remains on until the high-side switch turns on
again. The internal synchronous rectifier eliminates the
need for an external Schottky diode.
At inductor currents below 40mA (60mA), the MAX8884Y
(MAX8884Z) automatically switches to pulse-skipping
mode to improve light-load efficiency. Output voltage
ripple remains low at all loads, while the skip-mode
switching frequency remains ultrasonic down to 1mA
(typ) loads.
Voltage Positioning Load Regulation
The MAX8884Y/MAX8884Z step-down converters utilize
a unique feedback network. By taking a DC feedback
from the LX node through R1 in the
Block Diagram , the
usual phase lag due to the output capacitor is
removed, making the loop exceedingly stable and
allowing the use of very small ceramic output capaci-
tors. To improve the load regulation, resistor R3 is
included in the feedback (see the
Block Diagram ). This
configuration yields load regulation equal to half the
inductor’s series resistance multiplied by the load cur-
rent. This voltage positioning load regulation greatly
reduces overshoot during load transients.
SEL Output Voltage Selection
SEL is used to determine the output voltage of the buck
converter and LDO1. See Table 1.
Shutdown Mode
Drive BUCK_EN to logic-low to place the MAX8884Y/
MAX8884Z step-down converter in shutdown mode. In
shutdown, the control circuitry, internal switching
MOSFET, and synchronous rectifier turn off and LX
becomes high impedance.
The LDOs are individually enabled. Connect LDO1_EN
and LDO2_EN to GND or logic-low to place LDO1 and
LDO2 in shutdown mode. In shutdown, the outputs of
the LDOs are pulled to ground through an internal
100
Ω resistor.
When the step-down converter and all LDOs are in shut-
down, the MAX8884Y/MAX8884Z enter a very low-power
state, where the input current drops to 0.1μA (typ).
Step-Down Converter Soft-Start
The MAX8884Y/MAX8884Z step-down converter uses
internal soft-start circuitry to limit inrush current at startup,
reducing transients on the input source. Soft-start is partic-
ularly useful for supplies with high output impedance such
as Li+ and alkaline cells. See the soft-start waveforms in
the
Typical Operating Characteristics .
Thermal Shutdown
Thermal shutdown limits total power dissipation in the
MAX8884Y/MAX8884Z. If the junction temperature
exceeds +160°C, thermal shutdown circuitry turns off
the MAX8884Y/MAX8884Z, allowing the ICs to cool.
The ICs turn on and begin soft-start after the junction
temperature cools by 20°C. This results in a pulsed out-
put during continuous thermal-overload conditions.
Applications Information
Output Voltages
The MAX8884Y/MAX8884Z DC-DC step-down convert-
er sets the BUCK and LDO1 output voltage based on
the state of SEL. See Table 1.
Contact the factory for other output voltage options.
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
MAX8884Y/MAX8884Z 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 MAX8884Y operates with a switching frequency of
2MHz and utilizes a 2.2μH inductor. The MAX8884Z
operates with a switching frequency of 4MHz and uti-
lizes a 1μH inductor. The higher switching frequency of
the MAX8884Z allows the use of physically smaller
inductors at the cost of lower efficiency. The lower
switching frequency of the MAX8884Y results in greater
efficiency at the cost of a physically larger inductor.
See the
Typical Operating Characteristics for efficiency
graphs for both the MAX8884Y and the MAX8884Z.
V V
I R
I load cu
BUCK BUCK NO LOAD
LOAD DCR
LOAD
= −
×
=
_ _
2
rrrent
R DC impedance of inductor
V
DCR
BUCK NO LO
=
_ _ A AD
V or V depending on SEL = 1 2 1 8 . .
SEL
BUCK CONVERTER
OUTPUT VOLTAGE
(V)
LDO1
OUTPUT VOLTAGE
(V)
AGND 1.2 1.8
IN1_ 1.8 2.8
Table 1. SEL Output Voltage Selection