LTC3543 Datasheet, PDF (9/16 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

U
OPERATIO
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant-
frequency architectures by preventing subharmonic
oscillations at high duty cycles. It is accomplished in-
ternally by adding a compensating ramp to the inductor
current signal at duty cycles in excess of 40%. Normally,
this results in a reduction of maximum inductor peak
current for duty cycles >40%; however, the LTC3543
uses a patent-pending scheme that counteracts this com-
pensating ramp, allowing the maximum inductor peak
current to remain unaffected throughout all duty cycles.
LTC3543
1500
1400
1300
1200
1100
1000
900
800
700
600
500
2.5
V_OUT = 1.2V
V_OUT = 1.5V
3.0 3.5 4.0 4.5
INPUT VOLTAGE (V)
5.0 5.5
3543 F02
Figure 2. Maximum Output Current vs Input Voltage (V_IN)
APPLICATIO S I FOR ATIO
The basic LTC3543 application circuit is shown on the
front page of this datasheet. External component selec-
tion is driven by the load requirement and begins with the
selection of L followed by C_IN and C_OUT.
Inductor Selection
For most applications, the value of the inductor will fall
in the range of 1µH to 4.7µH. Its value is chosen based
on the desired ripple current. Large value inductors
lower ripple current, and small value inductors result in
higher ripple currents. Higher V_IN or V_OUT also increases
the ripple current, as shown in Equation 1. A reasonable
starting point for setting ripple current is ΔI_L = 130mA
20% I_LOADMAX.
∆I_L
=
V_OUT
f •L
•
⎛
⎝⎜
1−
V_OUT ⎞
V_IN ⎠⎟
(1)
The DC current rating of the inductor should be at least
equal to the maximum load current plus half the ripple
current to prevent core saturation. Thus, a 665mA rated
inductor should be enough for most applications (600mA
+ 65mA). For better efficiency, choose a low DC-resistance
inductor.
The inductor value also has an effect on Burst Mode opera-
tion. The transition to low current operation begins when
the inductor current peaks fall to approximately 100mA.
Lower inductor values (higher ΔI_L) will cause this to occur
at lower load currents, which can cause a dip in efficiency
in the upper range of low current operation. In Burst Mode
operation, lower inductance values will cause the burst
frequency to increase.
Inductor Core Selection
Different core materials and shapes will change the
size/current and price/current relationship of an
inductor. Toroid or shielded pot cores in ferrite or
permalloy materials are small and don’t radiate much
energy, but generally cost more than powdered iron core
inductors with similar electrical characteristics. The choice
of which style inductor to use often depends more on the
price vs size requirements, and any radiated field/EMI re-
quirements, than on what the LTC3543 requires to operate.
Table 1 shows some typical surface mount inductors that
work well in LTC3543 applications.
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle V_OUT/V_IN. To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
C_IN
required
I_RMS
≅
I_OMAX
•
[V_OUT
(V_IN –V_OUT
V_IN
)]^1/2
(2)
3543f
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