LTC3543_15 Datasheet, PDF (12/20 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIONS INFORMATION
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_O(MAX)
•
[V_OUT(V_IN –V_OUT
V_IN
)]^1/2
(2)
This formula has a maximum of V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Always consult the manufacturer if there
are any questions.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple ΔV_OUT is determined by:
ΔV_OUT
≅
ΔI_L
⎛
⎝⎜
ESR
+
1
8 fC_OUT
⎞
⎠⎟
(3)
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ΔI_L increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice
is the AVX TPS series of surface mount tantalum. These
are specially constructed and tested for low ESR so they
give the lowest ESR for a given volume. Other capacitor
types include the Sanyo POSCAP, the Kemet T510 and T495
series, and the Sprague 593D and 595D series. Consult
the manufacturer for other specific recommendations.
C1 Selection
When spread spectrum operation is enabled, the fre-
quency of the LTC3543 is randomly varied over the
range of frequencies between 2MHz and 3MHz. In
this case, a capacitor should be connected between
the CAP pin and GND to smooth out the changes in fre-
quency. This not only provides a smoother frequency spec-
trum but also ensures that the switching regulator remains
stable by preventing abrupt changes in frequency.
When the PLL mode is enabled, if the external clock fre-
quency is greater than the internal oscillator’s frequency
(OSC), then current is sourced continuously, pulling up the
voltage on the CAP pin. If the external clock frequency is
less than OSC, current is sunk continuously, pulling down
the voltage on the CAP pin. When the external and internal
frequencies are the same but exhibit a phase difference,
current pulses (sourcing or sinking) are used for an amount
of time corresponding to the phase difference. The current
pulses adjust the voltage on the CAP pin until the phase
and frequency of the internal and external oscillators are
identical. At the stable operating point, the CAP pin is high
impedance and the external capacitor holds the voltage.
The external cap is used by the PLL’s loop filter to help
smooth out the voltage change and provide a stable input
to the voltage-controlled oscillator. The value of C1 will
determine how fast the loop acquires lock. Typically C1
is 1nF to 10nF in PLL mode. A value of 2.2nF is suitable
in most applications.
3543fa
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