AAT2514_08 Datasheet, PDF (12/14 Pages) Advanced Analogic Technologies – Dual Channel 600mA Step-Down Converter

SwitchReg^TM
PRODUCT DATASHEET
AAT2514
Dual Channel 600mA Step-Down Converter
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_OUT
needed to limit the ripple to ∆V_O, V peak-to-peak is:
ESR ≤ ^ΔVO
ΔI_L
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_L, minus the output current,
I_O. The RMS value of the ripple current flowing in the
output capacitance (continuous inductor current mode
operation) is given by:
I_RMS = ΔI_L ·
3
6
ΔI_L · 0.289
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_OUT is determined by:
ΔV_OUT ≤
V_OUT ·
V_IN
(V_IN -
· f_OSC
V_OUT)
·L
·
⎛⎝_ESR
+
8
·
1
f_OSC
·
⎞
C3⎠
A 10μF ceramic capacitor can satisfy most applications.
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_DS(ON) characteristics of the
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:
P_TOTAL
=
I_O2
·
(R_DSON(HS)
·
V_O
+ R_DSON(LS)
V_IN
·
[V_IN
-
V_O])
+ (t_sw · F · 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(HS) + 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 temperature can be
derived from the θ_JA for the MSOP-10 or DFN-10 pack-
ages, which is 45°C/W.
T_J(MAX) = P_TOTAL · Θ_JA + T_AMB
Manufacturer
Murata
Murata
Murata
Part Number
GRM219R60J106KE19
GRM21BR60J226ME39
GRM1551X1E220JZ01B
Value
10μF
22μF
22pF
Voltage (V)
6.3
6.3
25
Table 3: Typical Surface Mount Capacitors.
Temp. Co.
X5R
X5R
JIS
Case
0805
0805
0402
12
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2514.2008.02.1.1