LTC3546_15 Datasheet, PDF (16/30 Pages) Linear Technology – Dual Synchronous, 3A/1A or 2A/2A Configurable Step-Down DC/DC Regulator

LTC3546
Applications Information
the maximum RMS current. The RMS current calculation
is different if the part is used in-phase or out-of-phase.
For in-phase, there are two different equations:
V_OUT1 > V_OUT2:
I_RMS =
( ) ( ) ^{2•I1•I2•D2(1–D1)+I22 D2–D22 +I12 D1–D12}
V_OUT2 > V_OUT1:
I_RMS =
( ) ( ) ^{2•I1•I2•D1(1–D2)+I22 D2–D22 +I12 D1–D12}
Where:
D1= ^VOUT1 and D2 = ^VOUT2
V_IN
V_IN
When D1 = D2, then the equation simplifies to:
I_RMS = (I1+I2) D(1–D)
or
I_RMS = (I1+I2)
( ) ^V_OUT ^V_IN ^{– V}_OUT
V_IN
where the maximum average output currents I1 and
I2 equals the peak current minus half the peak-to-peak
ripple current:
I1=
I_LIM1
–
ΔI_L1
2
I2
=
I_LIM2
–
ΔI_L2
2
These formula have a maximum at V_IN = 2V_OUT, where
I_RMS = (I1 + I2)/2. This simple worst-case is commonly
used to determine the worst-case I_RMS.
For out-of-phase (PHASE pin is at ground), the ripple
current can be lower than the in-phase.
In the out-of-phase case, the maximum I_RMS does not oc-
cur when V_OUT1 = V_OUT2. The maximum typically occurs
when V_OUT1 – V_IN/2 = V_OUT2 and when V_OUT2 – V_IN/2 =
V_OUT1. As a good rule of thumb, the amount of worst-case
ripple is about 75% of the worst-case ripple in the in-phase
mode. Note, that when V_OUT1 = V_OUT2 = V_IN/2 and I₁ = I₂,
the ripple is at its minimum.
Note that capacitor manufacturer’s ripple current 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_OUT1 and C_OUT2) Selection
The selection of C_OUT1 and C_OUT2 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
+
8
•
f_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.
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 tantalum
capacitors are all available in surface mount packages.
The OS-CON semiconductor dielectric capacitor avail-
able from Sanyo has the lowest ESR(size) product of any
aluminum electrolytic at a somewhat higher price. Special
polymer capacitors, such as Sanyo POSCAP, offer very
low ESR, but have a lower capacitance density than other
types. Tantalum capacitors have the highest capacitance
3546fc
16
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