AAT2514_08 ANALOGICTECH [Advanced Analogic Technologies], AAT2514_08 Datasheet - Page 12

• Manufacturer Part Number: AAT2514_08
• Description: Dual Channel 600mA Step-Down Converter
• Manufacturer: ANALOGICTECH [Advanced Analogic Technologies]

In many practical designs, to get the required ESR, a
capacitor with much more capacitance than is needed
must be selected. For both continuous or discontinuous
inductor current mode operation, the ESR of the C
needed to limit the ripple to ∆V
Ripple current flowing through a capacitor’s ESR causes
power dissipation in the capacitor. This power dissipation
causes a temperature increase internal to the capacitor.
Excessive temperature can seriously shorten the expect-
ed life of a capacitor. Capacitors have ripple current rat-
ings that are dependent on ambient temperature and
should not be exceeded. The output capacitor ripple cur-
rent is the inductor current, I
I
output capacitance (continuous inductor current mode
operation) is given by:
ESL can be a problem by causing ringing in the low
megahertz region but can be controlled by choosing low
ESL capacitors, limiting lead length (PCB and capacitor),
and replacing one large device with several smaller ones
connected in parallel.
In conclusion, in order to meet the requirement of out-
put voltage ripple small and regulation loop stability,
ceramic capacitors with X5R or X7R dielectrics are rec-
ommended due to their low ESR and high ripple current
ratings. The output ripple V
A 10μF ceramic capacitor can satisfy most applications.
12
SwitchReg
O
. The RMS value of the ripple current flowing in the
Δ V
Manufacturer
OUT
Murata
Murata
Murata
≤
V
TM
OUT
I
V
RMS
IN
· (V
· f
= ΔI
OSC
IN
ESR ≤
- V
L
· L
·
OUT
6
3
OUT
L
)
, minus the output current,
ΔV
GRM1551X1E220JZ01B
GRM21BR60J226ME39
ΔI
· ESR +
ΔI
GRM219R60J106KE19
O
is determined by:
⎛
⎝
L
L
, V peak-to-peak is:
O
Part Number
· 0.289
Table 3: Typical Surface Mount Capacitors.
8 · f
OSC
1
w w w . a n a l o g i c t e c h . c o m
· C3
⎞
⎠
OUT
Value
Dual Channel 600mA Step-Down Converter
10μF
22μF
22pF
Thermal Calculations
There are three types of losses associated with the
AAT2514 step-down converter: switching losses, conduc-
tion losses, and quiescent current losses. Conduction
losses are associated with the R
power output switching devices. Switching losses are
dominated by the gate charge of the power output switch-
ing devices. At full load, assuming continuous conduction
mode(CCM), a simplified form of the losses is given by:
I
term t
verter switching losses.
For the condition where the step-down converter is in
dropout at 100% duty cycle, the total device dissipation
reduces to:
Since R
vary with input voltage, the total losses should be inves-
tigated over the complete input voltage range. Given the
total losses, the maximum junction temperature can be
derived from the θ
ages, which is 45°C/W.
Q
is the step-down converter quiescent current. The
P
TOTAL
sw
DS(ON)
is used to estimate the full load step-down con-
Voltage (V)
+ (t
=
, quiescent current, and switching losses all
I
O
P
sw
6.3
6.3
2
25
TOTAL
T
· (R
· F · I
J(MAX)
DSON(HS)
JA
= I
O
PRODUCT DATASHEET
for the MSOP-10 or DFN-10 pack-
= P
O
+ I
2
· R
TOTAL
Q
) · V
· V
DSON(HS)
O
· Θ
Temp. Co.
IN
+ R
DS(ON)
AAT2514
V
JA
IN
X5R
X5R
DSON(LS)
JIS
+ I
+ T
characteristics of the
Q
AMB
· V
2514.2008.02.1.1
· [V
IN
IN
- V
Case
0805
0805
0402
O
])