LTC3863 Datasheet, PDF (18/36 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller Wide Operating VIN Range: 3.5V to 60V

LTC3863
APPLICATIONS INFORMATION
The formula shows that the RMS current is greater than
the maximum I_OUT when V_OUT is greater than V_IN. Choose
capacitors with higher RMS rating with sufficient margin.
Note that ripple current ratings from capacitor manufac-
turers are often based on only 2000 hours of life, which
makes it advisable to derate the capacitor.
The selection of C_OUT is primarily determined by the ESR
required to minimize voltage ripple and load step transients.
The ∆V_OUT is approximately bounded by:
∆V_OUT
≤ I_{L(PEAK )}
•ESR +
I_OUT •D
f •C_OUT
where I_L(PEAK) is the peak inductor current and it’s given as:
( ) _{IL(PEAK) =} ^I_OUT
V_IN + | V_OUT | +V_D
V_IN
(( ⁾ ) ⁺
2
•
L
V_IN
•f•
•
| V_OUT | +V_D
V_IN + | V_OUT | +
V_D
Since I_L(PEAK) and D reach their maximum values at mini-
mum V_IN, the output voltage ripple is highest at minimum
V_IN and maximum I_OUT. Typically, once the ESR require-
ment is satisfied, the capacitance is adequate for filtering
and has the necessary RMS current rating.
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, specialty polymer, aluminum electrolytic
and ceramic capacitors are all available in surface mount
packages. Specialty polymer capacitors offer very low
ESR but have lower specific capacitance than other types.
Tantalum capacitors have the highest specific capacitance,
but it is important to only use types that have been surge
tested for use in switching power supplies. Aluminum
electrolytic capacitors have significantly higher ESR, but
can be used in cost-sensitive applications provided that
consideration is given to ripple current ratings and long-
term reliability. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient and
audible piezoelectric effects.
The high Q of ceramic capacitors with trace inductance
can also lead to significant ringing. When used as input
capacitors, care must be taken to ensure that ringing from
inrush currents and switching does not pose an overvolt-
age hazard to the power switch and controller. To dampen
input voltage transients, add a small 5μF to 40μF aluminum
electrolytic capacitor with an ESR in the range of 0.5Ω to
2Ω. High performance through-hole capacitors may also
be used, but an additional ceramic capacitor in parallel
is recommended to reduce the effect of lead inductance.
Discontinuous and Continuous Operation
The LTC3863 operates in discontinuous conduction (DCM)
until the load current is high enough for the inductor
current to be positive at the end of the switching cycle.
The output load current at the continuous/discontinuous
boundary, I_OUT(CDB), is given by the following equation:
( ) _{IOUT(CDB) =}
V_IN(MAX)2 • | V_OUT | +V_D
( ) ^{2 •L • f • V}IN(MAX)^{+ | V}_OUT ^{| +V}D
2
The continuous/discontinuous boundary is inversely
proportional to the inductor value. Therefore, if required,
I_OUT(CDB) can be reduced by increasing the inductor value.
External Soft-Start and Output Tracking
Start-up characteristics are controlled by the voltage on
the SS pin. When the voltage on the SS pin is less than
the internal 0.8V reference, the LTC3863 regulates the V_FB
pin voltage to the voltage on the SS pin. When the SS pin
is greater than the internal 0.8V reference, the V_FB pin
voltage regulates to the 0.8V internal reference. The SS
pin can be used to program an external soft-start function
or to allow V_OUT to track another supply during start-up.
3863f
18
For more information www.linear.com/3863