LTC3561 Просмотр технического описания (PDF) - Linear Technology
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LTC3561 Datasheet PDF : 16 Pages
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LTC3561
APPLICATIO S I FOR ATIO
piezoelectric effects. In addition, the high Q of ceramic
capacitors along with trace inductance can lead to signifi-
cant ringing. Other capacitor types include the Panasonic
specialty polymer (SP) capacitors.
In most cases, 0.1µF to 1µF of ceramic capacitors should
also be placed close to the LTC3561 in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now
becoming available in smaller case sizes. These are tempt-
ing for switching regulator use because of their very low
ESR. Unfortunately, the ESR is so low that it can cause
loop stability problems. Solid tantalum capacitor ESR
generates a loop “zero” at 5kHz to 50kHz that is instrumen-
tal in giving acceptable loop phase margin. Ceramic ca-
pacitors remain capacitive to beyond 300kHz and usually
resonate with their ESL before ESR becomes effective.
Also, ceramic caps are prone to temperature effects which
requires the designer to check loop stability over the
operating temperature range. To minimize their large
temperature and voltage coefficients, only X5R or X7R
ceramic capacitors should be used. A good selection of
ceramic capacitors is available from Taiyo Yuden, TDK and
Murata.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the VIN pin. At best, this ringing can
couple to the output and be mistaken as loop instability. At
worst, the ringing at the input can be large enough to
damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfill a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough to
support the load. The time required for the feedback loop
to respond is dependent on the compensation compo-
nents and the output capacitor size. Typically, 3 to 4 cycles
are required to respond to a load step, but only in the first
cycle does the output drop linearly. The output droop,
VDROOP, is usually about 2 to 3 times the linear drop of the
first cycle. Thus, a good place to start is with the output
capacitor size of approximately:
C OUT
≈
2.5
fO
∆IOUT
• VDROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely re-
quired to supply high frequency bypassing, since the
impedance to the supply is very low. A 10µF ceramic
capacitor is usually enough for these conditions.
Setting the Output Voltage
The LTC3561 develops a 0.8V reference voltage between
the feedback pin, VFB, and the signal ground as shown in
Figure 4. The output voltage is set by a resistive divider
according to the following formula:
VOUT ≈ 0.8V ⎛⎝⎜1+ RR21⎞⎠⎟
Keeping the current small (<5µA) in these resistors maxi-
mizes efficiency, but making them too small may allow
stray capacitance to cause noise problems and reduce the
phase margin of the error amp loop.
To improve the frequency response, a feed-forward ca-
pacitor CF may also be used. Great care should be taken to
route the VFB line away from noise sources, such as the
inductor or the SW line.
Shutdown and Soft-Start
The SHDN/RT pin is a dual purpose pin that sets the
oscillator frequency and provides a means to shut down
the LTC3561. This pin can be interfaced with control logic
in several ways, as shown in Figure 2(a) and Figure 2(b).
The ITH pin is primarily for loop compensation, but it can
also be used to increase the soft-start time. Soft-start
3561f
8
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