MAX8794ETB(2006) 데이터 시트보기 (PDF) - Maxim Integrated
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MAX8794ETB Datasheet PDF : 13 Pages
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Low-Voltage DDR Linear Regulator
For a step-voltage change at REFIN, the rate of change
of the output voltage is limited by the total output
capacitance, the current limit, and the load during the
transition. Adding a capacitor across REFIN and AGND
filters noise and controls the rate of change of the
REFIN voltage during dynamic transitions. With the
additional capacitance, the REFIN voltage slews
between the two set points with a time constant given
by REQ x CREFIN, where REQ is the equivalent parallel
resistance seen by the slew capacitor.
Operating Region and Power Dissipation
The maximum power dissipation of the MAX8794
depends on the thermal resistance of the 10-pin TDFN
package and the circuit board, the temperature differ-
ence between the die and ambient air, and the rate of
airflow. The power dissipated in the device is:
PSRC = ISRC x (VIN – VOUT)
PSINK = ISINK x VOUT
The resulting maximum power dissipation is:
PDIS(MAX)
=
TJ(MAX) - TA
θJC + θCA
where TJ(MAX) is the maximum junction temperature
(+150°C), TA is the ambient temperature, θJC is the
thermal resistance from the die junction to the package
case, and θCA is the thermal resistance from the case
through the PC board, copper traces, and other materi-
als to the surrounding air. For optimum power dissipa-
tion, use a large ground plane with good thermal
contact to the backside pad, and use wide input and
output traces.
When 1in2 of copper is connected to the device, the
maximum allowable power dissipation of a 10-pin TDFN
package is 1951mW. The maximum power dissipation is
derated by 24.4mW/°C above TA = +70°C. Extra copper
on the PC board increases thermal mass and reduces
thermal resistance of the board. Refer to the MAX8794
evaluation kit for a layout example.
The MAX8794 delivers up to 3A and operates with input
voltages up to 3.6V, but not simultaneously. High output
currents can only be achieved when the input-output
differential voltages are low (Figure 5).
Dropout Operation
A regulator’s minimum input-to-output voltage differen-
tial (dropout voltage) determines the lowest usable sup-
ply voltage. Because the MAX8794 uses an n-channel
pass transistor, the dropout voltage is a function of the
drain-to-source on-resistance (RDS(ON) = 0.25Ω max)
multiplied by the load current (see the Typical
Operating Characteristics):
SAFE OPERATING REGION
3.5
DROPOUT VOLTAGE
3.0 LIMITED
MAXIMUM CURRENT LIMIT
2.5
2.0
TA = 0°C TO +70°C
1.5
VIN(MAX) - VOUT(MIN)
1.0
0.5
0
0
TA = +100°C
0.5 1.0 1.5 2.0 2.5 3.0 3.5
INPUT-OUTPUT DIFFERENTIAL VOLTAGE (V)
Figure 5. Power Operating Region—Maximum Output Current
vs. Input-Output Differential Voltage
VDROPOUT = RDS(ON) x IOUT
For low output-voltage applications, the sink current is
limited by the output voltage and the RDS(ON) of the
MOSFET.
Input Capacitor Selection
Bypass IN to PGND with a 10µF or greater ceramic
capacitor. Bypass VCC to AGND with a 1µF ceramic
capacitor for normal operation in most applications.
Typically, the LDO is powered from the output of a
step-down controller (memory supply) that has addi-
tional bulk capacitance (polymer or tantalum) and dis-
tributed ceramic capacitors.
Output Capacitor Selection
The MAX8794 output stability is independent of the out-
put capacitance for COUT from 10µF to 220µF.
Capacitor ESR between 2mΩ and 50mΩ is needed to
maintain stability. Within the recommended capaci-
tance and ESR limits, the output capacitor should be
chosen to provide good transient response:
∆IOUT(P-P) x ESR = ∆VOUT(P-P)
where ∆IOUT(P-P) is the maximum peak-to-peak load-
current step (typically equal to the maximum source
load plus the maximum sink load), and ∆VOUT(P-P) is
the allowable peak-to-peak voltage tolerance.
Using larger output capacitance can improve efficiency
in applications where the source and sink currents
change rapidly. The capacitor acts as a reservoir for
the rapid source and sink currents, so no extra current
is supplied by the MAX8794 or discharged to ground,
improving efficiency.
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