AAT2556 Datasheet, PDF (17/27 Pages) Advanced Analogic Technologies – Battery Charger and Step-Down Converter for Portable Applications

DATA SHEET
AAT2556
Battery Charger and Step-Down Converter for Portable Applications
500
400
T_A = 60°C
300
T_A = 85°C
200
100
0
4.25 4.5 4.75 5 5.25 5.5 5.75 6 6.25 6.5 6.75
V_IN (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 associ-
ated with the R_DS(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:
P_TOTAL
=
I_O2
·
(R_DSON(H)
·
V_O
+
R_DSON(L)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F_S · I_O + I_Q) · V_IN
I_Q is the step-down converter quiescent current. The
term t_sw 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:
P_TOTAL = I_O2 · R_DSON(H) + I_Q · V_IN
Since R_DS(ON), quiescent current, and switching losses all
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range.
Given the total losses, the maximum junction tempera-
ture can be derived from the _JA for the TDFN33-12
package which is 50°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
Capacitor Selection
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 C_IN 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.
Step-Down Converter Input Capacitor (C3)
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 (V_PP) and solve
for C_IN. The calculated value varies with input voltage and
is a maximum when V_IN is double the output voltage.
C_IN =
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
⎛ V_PP
⎝ I_O
- ESR^⎞⎠ · F_S
V_O
V_IN
·
⎛⎝_{1 -}
V_O ⎞
V_IN ⎠
=
1
4
for
V_IN
=
2
·
V_O
1
C_IN(MIN) = _{⎛ VPP}
⎝ I_O
- ESR^⎞⎠ · 4 · F_S
Always examine the ceramic capacitor DC voltage coef-
ficient 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.
The maximum input capacitor RMS current is:
V_O
V_IN
· ^⎛⎝1 -
V_O ⎞
V_IN ⎠
=
D · (1 - D) =
0.5² = ¹
2
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