LTC3548A Datasheet, PDF (10/20 Pages) Linear Technology – Dual Synchronous 400mA/800mA, 2.25MHz Step-Down DC/DC Regulator

LTC3548A
APPLICATIONS INFORMATION
Table 1. Representative Surface Mount Inductors
MANU-
MAX DC
FACTURER PART NUMBER VALUE CURRENT DCR HEIGHT
Taiyo
Yuden
CB2016T2R2M
CB2012T2R2M
CB2016T3R3M
2.2μH
2.2μH
3.3μH
510mA
530mA
410mA
0.13Ω 1.6mm
0.33Ω 1.25mm
0.27Ω 1.6mm
Panasonic ELT5KT4R7M
4.7μH 950mA 0.2Ω 1.2mm
Sumida CDRH2D18/LD 4.7μH 630mA 0.086Ω 2mm
Murata LQH32CN4R7M23 4.7μH 450mA 0.2Ω 2mm
Taiyo
Yuden
NR30102R2M
NR30104R7M
2.2μH 1100mA 0.1Ω 1mm
4.7μH 750mA 0.19Ω 1mm
FDK
FDKMIPF2520D 4.7μH 1100mA 0.11Ω 1mm
FDKMIPF2520D 3.3μH 1200mA 0.1Ω 1mm
FDKMIPF2520D 2.2μH 1300mA 0.08Ω 1mm
TDK
VLF3010AT4R7- 4.7μH 700mA 0.28Ω 1mm
MR70
VLF3010AT3R3- 3.3μH 870mA 0.17Ω 1mm
MR87
VLF3010AT2R2- 2.2μH 1000mA 0.12Ω 1mm
M1R0
Input Capacitor (C_IN) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V_OUT/V_IN.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
ΔV_OUT
≈
ΔI_L
⎛
⎝⎜
ESR
+
8f_O
1
C_OUT
⎞
⎠⎟
where the maximum average output current I_MAX equals
the peak current minus half the peak-to-peak ripple cur-
rent, I_MAX = I_LIM – ΔI_L/2.
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case is commonly used to
design because even significant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
V_IN for high frequency decoupling, when not using an all
ceramic capacitor solution.
Output Capacitor (C_OUT) Selection
The selection of C_OUT is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfied, the capacitance
is adequate for filtering. The output ripple (ΔV_OUT) is
determined by:
ΔV_OUT
≈
ΔI_L
⎛
⎝⎜
ESR
+
8f_O
1
C_OUT
⎞
⎠⎟
where f_O = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI_L increases
with input voltage. With ΔI_L = 0.3 • I_LIM the output ripple
will be less than 100mV at maximum V_IN and f_O = 2.25MHz
with:
ESR_COUT < 150mΩ
Once the ESR requirements for C_OUT have been met, the
RMS current rating generally far exceeds the I_RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolytic, special polymer, ceramic and dry
tantulum capacitors are all available in surface mount
packages. The OS-CON semiconductor dielectric capacitor
available from Sanyo has the lowest ESR (size) product
of any aluminum electrolytic at a somewhat higher price.
Special polymer capacitors, such as Sanyo POSCAP, of-
fer very low ESR, but have a lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density. However, they also have a larger
ESR and it is critical that they are surge tested for use
in switching power supplies. An excellent choice is the
3548af
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