LT3667EUDD View Datasheet(PDF) - Linear Technology
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Description
Manufacturer
LT3667EUDD Datasheet PDF : 34 Pages
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LT3667
APPLICATIONS INFORMATION
where fSW is in MHz, and COUT1 is the recommended output
capacitance in μF. Use X5R or X7R types. This choice will
provide low output ripple and good transient response.
Transient performance can be improved with a higher value
capacitor if combined with a phase lead capacitor (typically
22pF) between the output and pin FB1. Note that a larger
phase lead capacitor should be used with a large output
capacitor. A lower value of output capacitor can be used to
save space and cost but transient performance will suffer.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. Table 3 lists several capacitor
vendors.
Table 3: Capacitor Vendors
VENDOR
Panasonic
Kemet
Sanyo
Murata
AVX
Taiyo Yuden
URL
www.panasonic.com
www.kemet.com
www.sanyovideo.com
www.murata.com
www.avxcorp.com
www.taiyo-yuden.com
Low Ripple Burst Mode Operation
To enhance efficiency at light loads, the LT3667 oper-
ates in low ripple Burst Mode operation which keeps
the output capacitor charged to the proper voltage while
minimizing the input quiescent current. During Burst Mode
operation, the LT3667 delivers single cycle bursts of
current to the output capacitor followed by sleep periods
where the output power is delivered to the load by the
output capacitor. Because the LT3667 delivers power
to the output with single, low current pulses, the output
ripple is kept below 5mV for a typical application. As the
load current decreases towards a no load condition, the
percentage of time that the LT3667 operates in sleep mode
increases and the average input current is greatly reduced
resulting in high efficiency even at very low loads. Note
that during Burst Mode operation, the switching frequency
will be lower than the programmed switching frequency.
At higher output loads (above ~50mA for the front page
application) the LT3667 will be running at the frequency
programmed by the RT resistor, and will be operating in
standard PWM mode. The transition between PWM and
low ripple Burst Mode operation is seamless, and will not
disturb the output voltage.
Audible Noise
Ceramic capacitors are small, robust and have very
low ESR. However, ceramic capacitors can sometimes
cause problems when used with the LT3667 due to their
piezoelectric nature. When in Burst Mode operation, the
LT3667’s switching frequency depends on the load current,
and at very light loads the LT3667 can excite the ceramic
capacitor at audio frequencies, generating audible noise.
Since the LT3667 operates at a lower current limit during
Burst Mode operation, the noise is typically very quiet. If
this is unacceptable, use a high performance tantalum or
electrolytic capacitor at the output.
VSW
5V/DIV
IL
100mA/DIV
VOUT1
5mV/DIV
ILOAD = 10mA
1µs/DIV
FRONT PAGE APPLICATION
3667 F01
Figure 1. Burst Mode Operation
For more information www.linear.com/LT3667
3667fb
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