LTC3355_15 Datasheet, PDF (15/20 Pages) Linear Technology – 20V 1A Buck DC/DC with Integrated SCAP Charger and Backup Regulator

LTC3355
APPLICATIONS INFORMATION
Audible Noise
Ceramic capacitors are small, robust and have very low
ESR. However, ceramic capacitors can sometimes cause
problems when used with switching regulators. Both the
buck and boost can run in Burst Mode operation and the
switching frequency will depend on the load current which
at very light loads can excite the ceramic capacitors at
audio frequencies, generating audible noise. Since the
buck and boost operate at lower current limits in Burst
Mode operation, the noise is typically very quiet. Use a
high performance tantalum or electrolytic at the output if
the noise level is unacceptable.
Buck Soft-Start
When the buck is enabled soft-start is engaged. Soft-start
reduces the inrush current by taking more time to reach
the final output voltage. This is achieved by limiting the
buck output current over a 1ms period.
Boost Rectifier Diode
A Schottky rectifier diode (D2 in the Block Diagram) is
recommended for the boost rectifier diode. The diode
should have low forward drop at the peak operating
current, low reverse current and fast reverse recovery
times. The current rating should take into account power
dissipation as well as output current requirements. The
diode current rating should be equal to or greater than the
average forward current which is normally equal to the
output current. The reverse breakdown voltage should be
greater than the V_OUT voltage plus the peak ringing voltage
that is generated at the SW2 pin. Generally higher reverse
breakdown diodes will have lower reverse currents. Refer
to Table 3 for Schottky diode vendors.
Table 3. Schottky Diode Vendors
PART NUMBER
V_F AT 1A V_F AT 2A
V_R (V) I_AVE (A) (mV) (mV)
Diodes Inc.
B130
30
1
460
B230
30
2
430
Rohm
RSX201VA-30
30
1
360
Vishay
VS-20MQ060
60 2.1
I_R AT 5V
85°C (µA)
20
100
600
Boost Inductor L2 Selection and
Maximum Output Current
The boost inductor L2 should be 3.3µH to ensure fast
transfer of power from the buck to the boost after a V_IN
power outage. Refer to Table 1 for inductor vendors.
Boost Frequency Compensation
The LTC3355 boost switching regulator uses current mode
control to regulate V_OUT. This simplifies loop compensa-
tion and ceramic output capacitors can be used. The boost
regulator does not require the ESR of the output capaci-
tor for stability. Frequency compensation is provided by
the components connected to the V_CBST pin. Generally a
capacitor (C_C) and resistor (R_C) in series to ground are
used as shown in the Block Diagram.
Loop compensation determines the stability and transient
performance. Optimizing the design of the compensation
network depends on the application and type of output
capacitor. A practical approach is to start with one of the
circuits in this data sheet that is similar to your applica-
tion and tune the compensation network to optimize the
performance. Stability should then be checked across all
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