MAX1510_11 Datasheet, PDF (11/13 Pages) Maxim Integrated Products – Low-Voltage DDR Linear Regulator Thermal Fault Protection

Low-Voltage DDR Linear Regulators
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 R_EQ x C_REFIN, where R_EQ is the equivalent parallel
resistance seen by the slew capacitor.
Operating Region and Power Dissipation
The maximum power dissipation of the MAX1510/
MAX17510 depends on the thermal resistance of the 10-
pin TDFN package and the circuit board, the tempera-
ture difference between the die and ambient air, and the
rate of airflow. The power dissipated in the device is:
P_SRC = I_SRC x (V_IN – V_OUT)
P_SINK = I_SINK x V_OUT
The resulting maximum power dissipation is:
P_DIS(MAX)
=
T_J(MAX) - T_A
θ_JC + θ_CA
where T_J(MAX) is the maximum junction temperature
(+150°C), T_A is the ambient temperature, θ_JC is the ther-
mal resistance from the die junction to the package case,
and θ_CA is the thermal resistance from the case through
the PCB, copper traces, and other materials to the sur-
rounding air. For optimum power dissipation, use a large
ground plane with good thermal contact to the backside
pad, and use wide input and output traces.
When 1 square inch of copper is connected to the
device, the maximum allowable power dissipation of a
10-pin DFN package is 1951mW. The maximum power
dissipation is derated by 24.4mW/°C above T_A = +70°C.
Extra copper on the PCB increases thermal mass and
reduces thermal resistance of the board. Refer to the
MAX1510 evaluation kit for a layout example.
The MAX1510/MAX17510 deliver up to 3A and oper-
ates with input voltages up to 3.6V, but not simultane-
ously. 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 MAX1510/MAX17510 use an
n-channel pass transistor, the dropout voltage is a func-
tion of the drain-to-source on-resistance (R_DS(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
1s RMS
LIMIT
2.0
T_A = 0^°C TO +70^°C
100s RMS
1.5
LIMIT
V_IN(MAX) - V_OUT(MIN)
1.0
0.5
T_A = +100^°C
0
0 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
V_DROPOUT = R_DS(ON) x I_OUT
For low output-voltage applications, the sink current is
limited by the output voltage and the R_DS(ON) of the
MOSFET.
Input Capacitor Selection
Bypass IN to PGND with a 10µF or greater ceramic
capacitor. Bypass V_CC 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 MAX1510/MAX17510 output stability is indepen-
dent of the output capacitance for C_OUT from 10µF to
220µF. Capacitor ESR between 2mΩ and 50mΩ is
needed to maintain stability. Within the recommended
capacitance and ESR limits, the output capacitor
should be chosen to provide good transient response:
ΔI_OUT(P-P) x ESR = ΔV_OUT(P-P)
where ΔI_OUT(P-P) is the maximum peak-to-peak load-
current step (typically equal to the maximum source
load plus the maximum sink load), and ΔV_OUT(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 MAX1510/MAX17510 or discharged
to ground, improving efficiency.
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