MAX1586A_08 Datasheet, PDF (24/30 Pages) Maxim Integrated Products – High-Efficiency, Low-IQ PMICs with Dynamic Core for PDAs and Smart Phones

High-Efficiency, Low-I_Q PMICs with
Dynamic Core for PDAs and Smart Phones
The LSB of the address word is the read/write (R/W) bit.
R/W indicates whether the master is writing or reading
(RD/W 0 = write, RD/W 1 = read). The MAX1586/
MAX1587 only support the SEND BYTE format; there-
fore, RD/W is required to be 0.
After receiving the proper address, the MAX1586/
MAX1587 issue an ACK by pulling SDA low for one
clock cycle. The MAX1586/MAX1587 have two user-
programmed addresses (Table 3). Address bits A7
through A2 are fixed, while A1 is controlled by SRAD.
Connecting SRAD to GND sets A1 = 0. Connecting
SRAD to IN sets A1 = 1.
V3 Output Ramp-Rate Control
When V3 is dynamically changed with the serial inter-
face, the output voltage changes at a rate controlled by
a capacitor (C_RAMP) connected from RAMP to ground.
The voltage change is a conventional RC exponential
described by:
Vo(t) = Vo(0) + dV(1 – exp(-t/(100kΩ C_RAMP)))
A useful approximation is that it takes approximately 2.2
RC time constants for V3 to move from 10% to 90% of
the voltage difference. For C_RAMP = 1500pF, this time
is 330µs. For 1V to 1.3V change, this equates to
1mV/µs. See the Typical Operating Characteristics for
examples of different ramp-rate settings.
The maximum capacitor value that can be used at
RAMP is 2200pF. If larger values are used, the V3 ramp
rate is still controlled according to the above equation,
but when V3 is first activated, POK indicates an “in reg-
ulation” condition before V3 reaches its final voltage.
The RAMP pin is effectively the reference for REG3.
FB3 regulates to 1.28 times the voltage on RAMP.
Design Procedure
Setting the Output Voltages
The outputs V1 and V2 have preset output voltages, but
can also be adjusted using a resistor voltage-divider. To
set V1 to 3.3V, connect FB1 to GND. V2 can be preset to
1.8V or 2.5V on the MAX1586A and MAX1587A. To set
V2 to 1.8V on the MAX1586A and MAX1587A, connect
FB2 to IN. To set to 2.5V, connect FB2 to GND. V2 can
preset to 3.3V or 2.5V on the MAX1587B. To set V2 to
3.3V on the MAX1587B, connect FB2 to IN. To set to
2.5V, connect FB2 to GND.
To set V1 or V2 to other than the preset output voltages,
connect a resistor voltage-divider from the output volt-
age to the corresponding FB input. The FB_ input bias
current is less than 100nA, so choose the low-side
(FB_-to-GND) resistor (R_L) to be 100kΩ or less. Then cal-
culate the high-side (output-to-FB_) resistor (R_H) using:
R_H = R_L [(V_OUT/1.25) – 1]
The V3 (VCC_CORE) output voltage is set from 0.7V to
1.475V in 25mV steps by the I²C serial interface. See
the Serial Interface section for details.
Linear regulator V4 provides a fixed 1.3V output volt-
age. Linear regulator V5 provides a fixed 1.1V output
voltage. V4 and V5 voltages are not adjustable.
The output voltage of linear regulator V6 (VCC_USIM) is
set to 0V, 1.8V, 2.5V, or 3.0V by the I²C serial interface.
See the Serial Interface section for details.
Linear regulator V7 (VCC_BATT) tracks the voltage at
V1 as long as ON1 is high and V1 is in regulation. When
ON1 is low or V1 is not in regulation, V7 switches to the
backup battery (V_BKBT).
Inductor Selection
The external components required for the step-down
are an inductor, input and output filter capacitors, and a
compensation RC network.
The MAX1586/MAX1587 step-down converters provide
best efficiency with continuous inductor current. A rea-
sonable inductor value (L_IDEAL) is derived from:
L_IDEAL = [2(V_IN) x D(1 - D)]/(I_OUT(MAX) x f_OSC)
This sets the peak-to-peak inductor current at 1/2 the
DC inductor current. D is the duty cycle:
D = V_OUT/ V_IN
Given L_IDEAL, the peak-to-peak inductor ripple current
is 0.5 x I_OUT. The peak inductor current is 1.25 x
I_OUT(MAX). Make sure the saturation current of the
inductor exceeds the peak inductor current and the
rated maximum DC inductor current exceeds the maxi-
mum output current (I_OUT(MAX)). Inductance values
larger than L_IDEAL can be used to optimize efficiency or
to obtain the maximum possible output current. Larger
inductance values accomplish this by supplying a
given load current with a lower inductor peak current.
Typically, output current and efficiency are improved
for inductor values up to about two times L_IDEAL. If the
inductance is raised too much, however, the inductor
size may become too large, or the increased inductor
resistance may reduce efficiency more than the gain
derived from lower peak current.
Smaller inductance values allow smaller inductor sizes,
but also result in larger peak inductor current for a
given load. Larger output capacitance may then be
needed to suppress the increase in output ripple
caused by larger peak current.
Capacitor Selection
The input capacitor in a DC-DC converter reduces cur-
rent peaks drawn from the battery or other input power
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