PRODUCT DATASHEET
AAT2552178
SystemPowerTM
Total Power Solution for Portable Applications
Capacitor Selection
500
450
400
350
300
250
200
150
TA = 25°C
Linear Regulator Input Capacitor (C6)
An input capacitor greater than 1μF will offer superior
input line transient response and maximize power sup-
ply ripple rejection. Ceramic, tantalum, or aluminum
electrolytic capacitors may be selected for CIN. There is
no specific capacitor ESR requirement for CIN. However,
for 300mA LDO regulator output operation, ceramic
capacitors are recommended for CIN due to their inher-
ent capability over tantalum capacitors to withstand
input current surges from low impedance sources such
as batteries in portable devices.
TA = 60°C
TA = 45°C
TA = 85°C
100
50
0
4.25 4.5 4.75
5
5.25 5.5 5.75
6
6.25 6.5 6.75
7
VIN (V)
Figure 4: Maximum Charging Current Before
Thermal Cycling Becomes Active.
Battery Charger Input Capacitor (C1)
In general, it is good design practice to place a decou-
pling capacitor between the ADP pin and GND. An input
capacitor in the range of 1μF to 22μF is recommended.
If the source supply is unregulated, it may be necessary
to increase the capacitance to keep the input voltage
above the under-voltage lockout threshold during device
enable and when battery charging is initiated. If the
adapter input is to be used in a system with an external
power supply source, such as a typical AC-to-DC wall
adapter, then a CIN capacitor in the range of 10μF should
be used. A larger input capacitor in this application will
minimize switching or power transient effects when the
power supply is “hot plugged” in.
There are three types of losses associated with the step-
down converter: switching losses, conduction losses, and
quiescent current losses. Conduction losses are associ-
ated with the RDS(ON) characteristics of the power output
switching devices. Switching losses are dominated by the
gate charge of the power output switching devices. At full
load, assuming continuous conduction mode (CCM), a
simplified form of the losses is given by:
IO2 · (RDSON(H) · VO + RDSON(L) · [VIN - VO])
PTOTAL
=
VIN
Step-Down Converter Input Capacitor (C6)
+ (tsw · FS · IO + IQ) · VIN
Select a 4.7μF to 10μF X7R or X5R ceramic capacitor for
the input. To estimate the required input capacitor size,
determine the acceptable input ripple level (VPP) and solve
for CIN. The calculated value varies with input voltage and
is a maximum when VIN is double the output voltage.
IQ is the step-down converter quiescent current. The
term tsw is used to estimate the full load step-down con-
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
VO
VIN
⎛
VO ⎞
VIN ⎠
· 1 -
⎝
CIN =
⎛ VPP
⎝ IO
⎞
PTOTAL = IO2 · RDSON(H) + IQ · VIN
- ESR ·FS
⎠
VO
VIN
⎛
VO ⎞
1
Since RDS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
· 1 -
=
for VIN = 2 · VO
⎝
VIN ⎠
4
1
CIN(MIN)
=
Given the total losses, the maximum junction tempera-
ture can be derived from the θJA for the TDFN34-16
package which is 50°C/W.
⎛ VPP
⎝ IO
⎞
⎠
- ESR · 4 · FS
Always examine the ceramic capacitor DC voltage coeffi-
cient characteristics when selecting the proper value. For
example, the capacitance of a 10μF, 6.3V, X5R ceramic
capacitor with 5.0V DC applied is actually about 6μF.
TJ(MAX) = PTOTAL · ΘJA + TAMB
w w w . a n a l o g i c t e c h . c o m
20
2552.2008.02.1.2
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
