ADP3401 Analog Devices, ADP3401 Datasheet - Page 10

• Manufacturer Part Number: ADP3401
• Description: GSM Power Management System
• Manufacturer: Analog Devices

ADP3401
Low Dropout Regulators (LDOs)
The ADP3401 high-performance LDOs are optimized for
their given functions by balancing quiescent current, dropout
voltage, line/load regulation, ripple rejection, and output
noise. 2.2 F tantalum or MLCC ceramic capacitors are
recommended for use with the digital and analog LDOs, and
0.22 F for the TCXO LDO.
Digital LDO (VCC)
The digital LDO (VCC) supplies all the digital circuitry in the
handset (baseband processor, baseband converter, external
memory, display, etc). The LDO has been optimized for very
low quiescent current (30 A maximum) at light loads as this
LDO is on at all times. This is due to both the structure of GSM
and a new clocking scheme used in the AD20msp425. Figure 17
shows how the digital current varies as a function of time.
Analog LDO (VCCA)
This LDO has the same features as the digital LDO. It has further-
more been optimized for good low frequency ripple rejection for use
with analog sections in order to reject the ripple coming from the RF
power amplifier. VCCA is rated to 130 mA load which is sufficient
to supply the complete analog section of a baseband converter such
as the AD6421/AD6425, including a 32
LDO and the TCXO LDO can be controlled by ANALOGON.
TCXO LDO (VTCXO)
The TCXO LDO is intended as a supply for the temperature-
compensated crystal oscillator, which needs its own ultralow noise
supply. The output current is rated to 5 mA for the TCXO LDO.
Figure 16. Total LDO Load Current vs. Temperature and VBAT
Figure 17. Digital Power as a Function of Time
MICROPROCESSOR
300
250
200
150
100
50
0
20
4-LAYER BOARD
JA
= 60 C/W
~50mA
START
~200 A
0
AMBIENT TEMPERATURE – C
~2ms
20
MICROPROCESSOR
STOP
0.5s TO 2s
40
VBAT = 5.5V
VBAT = 6V
earpiece. The analog
VBAT = 5V
60
VBAT = 7V
TIME
80
85
–10–
RTC LDO (VRTC)
The RTC LDO charges a rechargable coin cell to run the real-
time clock module. It has been targeted to charge Manganese
Lithium batteries such as the ML series (ML621/ML1220)
from Sanyo. The ML621 has a small physical size (6.8 mm
diameter) and a nominal capacity of 2.5 mAh, which yields
about 250 hours of backup time.
Figure 18 shows the use of VRTC with the Enhanced GSM
Processor which is a part of the AD20msp425 chipset.
The ADP3401 supplies current both for charging the coin cell and
for the RTC module when the digital supply is off. The nominal
charging voltage of 2.85 V ensures charging down to a main
battery voltage of 3.0 V. The inherent current limit of VRTC
ensures long cell life while the precise output voltage regulation
charges the cell to more than 90% of its capacity. In addition, it
features a very low quiescent current (10 A) since this LDO is
running all the time, even when the handset is switched off. It also
has reverse current protection with low leakage which is needed
when the main battery is removed and the coin cell supplies the
RTC module.
The RTC module has a built-in alarm which, when it expires,
will pull POWERONIN high, allowing an alarm function even if
the handset is switched off.
Reference Output (REFOUT)
The reference output is a low-noise, high-precision reference
with a guaranteed accuracy of 1.5% over temperature. The
reference can be fed to the baseband converter, such as the
AD6425, improving the absolute accuracy of the converters
from 5% to 1.5%. This significantly reduces calibration time
needed for the baseband converter during production.
SIM Interface
The SIM interface generates the needed SIM voltage—either 3 V
or 5 V, dependent on SIM type, and also performs the needed
logic level translation. Quiescent current is low, as the SIM card
will be powered all the time. Note that DATAIO and I/O have
integrated pull-up resistors as shown in Figure 19. See Table II for
the control logic of the charge pump output, VSIM.
Figure 18. Connecting VRTC and POWERONIN to the
AD20msp425 Chipset
ADP3401
PWRONIN
VRTC
CELL
COIN
VRTC
PWRON
ENHANCED
GSM PROCESSOR
(AD20msp425)
MODULE
RTC
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