AAT2554
Total Power Solution for Portable Applications
By substitution, we can derive the maximum
IQ is the step-down converter quiescent current.
The term tsw is used to estimate the full load step-
down converter switching losses.
charge current before reaching the thermal limit
condition (thermal cycling). The maximum charge
current is the key factor when designing battery
charger applications.
For the condition where the step-down converter is
in dropout at 100% duty cycle, the total device dis-
sipation reduces to:
(PD(MAX)
-
VIN
VIN - VBAT
· IOP)
ICH(MAX)
=
PTOTAL = IO2 · RDSON(H) + IQ · VIN
(TJ(MAX)
θJA
VIN - VBAT
- TA)
-
VIN · IOP
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.
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.
Given the total losses, the maximum junction tem-
perature can be derived from the θJA for the
TDFN34-16 package which is 50°C/W.
TJ(MAX)
=
PTOTAL
·
Θ
JA + TAMB
500
400
TA = 60°C
300
Capacitor Selection
Linear Regulator Input Capacitor (C7)
TA = 85°C
200
An input capacitor greater than 1µF will offer supe-
rior input line transient response and maximize
power supply ripple rejection. Ceramic, tantalum,
or aluminum electrolytic capacitors may be select-
ed for CIN. There is no specific capacitor ESR
requirement for CIN. However, for 300mA LDO reg-
ulator output operation, ceramic capacitors are rec-
ommended for CIN due to their inherent capability
over tantalum capacitors to withstand input current
surges from low impedance sources such as bat-
teries in portable devices.
100
0
4.25 4.5 4.75
5
5.25 5.5 5.75
6
6.25 6.5 6.75
VIN (V)
Figure 4: Maximum Charging Current Before
Thermal Cycling Becomes Active.
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:
Battery Charger Input Capacitor (C3)
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
IO2 · (RDSON(H) · VO + RDSON(L) · [VIN - VO])
PTOTAL
=
VIN
+ (tsw · FS · IO + IQ) · VIN
22
2554.2007.01.1.2
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