MAX1798_10 Datasheet, PDF (17/19 Pages) Maxim Integrated Products – CDMA Cellular/PCS System Power Supplies

CDMA Cellular/PCS System
Power Supplies
Applications Information
Capacitor Selection and
Regulator Stability
Use a 10µF low-ESR ceramic capacitor on the
MAX1798/MAX1798A/MAX1799/MAX1799A’s input if all
the supply inputs are connected together. Larger input
capacitance and lower ESR provide better supply noise
rejection and line-transient response. If IN1, IN2/3, and
IN4/5 are connected to different supply voltages,
bypass each input with a 4.7µF low-ESR ceramic
capacitor.
A minimum 4.7µF low-ESR ceramic capacitor is recom-
mended on OUT1, and a minimum 2.2µF low-ESR
ceramic capacitor is recommended on OUT2–5. The
MAX1798/MAX1798A/MAX1799/MAX1799A are stable
with output capacitors in the ESR range of 10mΩ to 1Ω.
Use larger capacitors to reduce 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. With
dielectrics such as Z5U and Y5V, it may be necessary
to use a minimum 4.7µF on OUT2–5 to ensure stability
at temperatures below -10°C. With X7R or X5R
dielectrics, 2.2µF should be sufficient at all operating
temperatures. Tantalum capacitors may cause instabili-
ty with the MAX1798/MAX1798A/MAX1799/MAX1799A
and are not recommended for this application.
Use a 0.01µF bypass capacitor at BP for low output-
voltage noise. Increasing the capacitance will slightly
decrease the output noise but will increase the startup
time. Values above 0.1µF provide no performance
advantage and are not recommended.
Line-Transient Considerations
The MAX1798/MAX1798A/MAX1799/MAX1799A are
designed to deliver low dropout voltages and low qui-
escent currents in battery-powered systems. Power-
supply rejection is >60dB at low frequencies and rolls
off above 10kHz. See the Power-Supply Rejection Ratio
(PSRR) vs. Frequency graph in the Typical Operating
Characteristics.
When operating from sources other than batteries,
improved supply noise rejection and transient response
can be achieved by increasing the values of the input
and output bypass capacitors and through passive fil-
tering techniques. The Typical Operating Character-
istics show the MAX1798/MAX1798A/MAX1799/
MAX1799A line- and load-transient responses.
Load-Transient Considerations
The MAX1798/MAX1798A/MAX1799/MAX1799A load-
transient response graphs (see Typical Operating
Characteristics) show three components of the output
response: the output capacitor’s ESR spike, the regula-
tor’s transient settling response, and the DC shift due to
the LDO’s load regulation. Increasing the output capaci-
tor’s value and decreasing the ESR reduce the over-
shoot.
Dropout Voltage
A regulator’s minimum input-output voltage differential
(dropout voltage) determines the lowest usable supply
voltage. In battery-powered systems, this determines
the useful end-of-life battery voltage. Because the
MAX1798/MAX1798A/MAX1799/MAX1799A use P-
channel MOSFET pass transistors, their dropout volt-
age is a function of drain-to-source on-resistance
(R_DS(ON)) multiplied by the load current. See the
Dropout Voltage (OUT1) vs. Load Current graph in the
Typical Operating Characteristics.
Ordering Information (continued)
PA RT
TEMP
RANGE
PIN-
PACKAGE
MAX1799ETP+ -40°C to +85°C 20 TQFN
MAX1799EUP+ -40°C to +85°C 20 TSSOP-EP
MAX1799AETP+ -40°C to +85°C 20 TQFN
MAX1799AEUP+ -40°C to +85°C 20 TSSOP-EP
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
INTER-
FACE
I²C
I²C
I²C
I²C
PROCESS: BiCMOS
Chip Information
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