AAT2556
Battery Charger and Step-Down
Converter for Portable Applications
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
(PD(MAX)
-
VIN
VIN - VBAT
· IOP)
ICH(MAX)
=
(TJ(MAX)
θJA
VIN - VBAT
- TA)
-
VIN · IOP
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
ICH(MAX)
=
In general, the worst condition is the greatest volt-
age drop across the IC, when battery voltage is
charged up to the preconditioning voltage thresh-
old. Figure 4 shows the maximum charge current in
different ambient temperatures.
PTOTAL = IO2 · RDSON(H) + IQ · VIN
Since RDS(ON), quiescent current, and switching
losses all vary with input voltage, the total losses
should be investigated over the complete input
voltage range.
500
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
TDFN33-12 package which is 50°C/W.
400
TA = 60°C
300
TA = 85°C
200
TJ(MAX)
=
PTOTAL
·
Θ
JA + TAMB
100
0
4.25 4.5 4.75
5
5.25 5.5 5.75
6
6.25 6.5 6.75
VIN (V)
Capacitor Selection
Battery Charger Input Capacitor (C1)
Figure 4: Maximum Charging Current Before
Thermal Cycling Becomes Active.
In general, it is good design practice to place a
decoupling 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 associated with the RDS(ON) characteris-
tics 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 (C3)
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (VPP) and solve for CIN. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
+ (tsw · FS · IO + IQ) · VIN
18
2556.2006.05.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
