AAT3218 Datasheet, PDF (13/18 Pages) List of Unclassifed Manufacturers – 150mA MicroPower™ High Performance LDO

AAT3218
150mA MicroPower™ High Performance LDO
Applications Information
High Peak Output Current Applications
Some applications require the LDO regulator to
operate at continuous nominal level with short
duration high current peaks. The duty cycles for
both output current levels must be taken into
account. To do so, one would first need to calcu-
late the power dissipation at the nominal continu-
ous level, then factor in the additional power dissi-
pation due to the short duration high current peaks.
For example, a 2.5V system using a AAT3218IGV-
2.5-T1 operates at a continuous 100mA load cur-
rent level and has short 150mA current peaks. The
current peak occurs for 378µs out of a 4.61ms peri-
od. It will be assumed the input voltage is 4.2V.
First the current duty cycle in percent must be cal-
culated:
% Peak Duty Cycle: X/100 = 378µs/4.61ms
% Peak Duty Cycle = 8.2%
The LDO Regulator will be under the 100mA load
for 91.8% of the 4.61ms period and have 150mA
peaks occurring for 8.2% of the time. Next, the
continuous nominal power dissipation for the
100mA load should be determined then multiplied
by the duty cycle to conclude the actual power dis-
sipation over time.
P_D(MAX) = (V_IN - V_OUT)I_OUT + (V_IN x I_GND)
P_D(100mA) = (4.2V - 2.5V)100mA + (4.2V x 150µA)
P_D(100mA) = 170.6mW
P_D(91.8%D/C) = %DC x P_D(100mA)
P_D(91.8%D/C) = 0.918 x 170.6mW
P_D(91.8%D/C) = 156.6mW
The power dissipation for 100mA load occurring for
91.8% of the duty cycle will be 156.6mW. Now the
power dissipation for the remaining 8.2% of the
duty cycle at the 150mA load can be calculated:
P_D(MAX) = (V_IN - V_OUT)I_OUT + (V_IN x I_GND)
P_D(150mA) = (4.2V - 2.5V)150mA + (4.2V x 150mA)
P_D(150mA) = 255.6mW
P_D(8.2%D/C) = %DC x P_D(150mA)
P_D(8.2%D/C) = 0.082 x 255.6mW
P_D(8.2%D/C) = 21mW
The power dissipation for 150mA load occurring for
8.2% of the duty cycle will be 21mW. Finally, the
3218.2004.02.1.0
two power dissipation levels can summed to deter-
mine the total true power dissipation under the var-
ied load.
P_D(total) = P_D(100mA) + P_D(150mA)
P_D(total) = 156.6mW + 21mW
P_D(total) = 177.6mW
The maximum power dissipation for the AAT3218
operating at an ambient temperature of 85°C is
211mW. The device in this example will have a total
power dissipation of 177.6mW. This is well within
the thermal limits for safe operation of the device.
Printed Circuit Board Layout
Recommendations
In order to obtain the maximum performance from
the AAT3218 LDO regulator, very careful attention
must be considered in regard to the printed circuit
board (PCB) layout. If grounding connections are
not properly made, power supply ripple rejection,
low output self noise and transient response can
be compromised.
Figure 1 shows a common LDO regulator layout
scheme. The LDO Regulator, external capacitors
(C_IN, C_OUT and C_BYP) and the load circuit are all con-
nected to a common ground plane. This type of lay-
out will work in simple applications where good
power supply ripple rejection and low self noise are
not a design concern. For high performance appli-
cations, this method is not recommended.
The problem with the layout in Figure 1 is the bypass
capacitor and output capacitor share the same
ground path to the LDO regulator ground pin along
with the high current return path from the load back
to the power supply. The bypass capacitor node is
connected directly to the LDO regulator internal ref-
erence, making this node very sensitive to noise or
ripple. The internal reference output is fed into the
error amplifier, thus any noise or ripple from the
bypass capacitor will be subsequently amplified by
the gain of the error amplifier. This effect can
increase noise seen on the LDO regulator output as
well as reduce the maximum possible power supply
ripple rejection. There is PCB trace impedance
between the bypass capacitor connection to ground
and the LDO regulator ground connection. When
the high load current returns through this path, a
small ripple voltage is created, feeding into the C_BYP
loop.
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