LTC3864 Datasheet, PDF (16/28 Pages) Linear Technology – 60V Low IQ Step-Down DC/DC Controller with 100% Duty Cycle Capability

LTC3864
APPLICATIONS INFORMATION
Since ∆I_L increases with input voltage, the output ripple
is highest at maximum input voltage. Typically, once the
ESR requirement 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 LTC3864 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:
I
OU T(CDB)
≅
(V_IN –
2•
V_OUT)( V_OUT+ V_F)
L • f • (V_IN + V_F)
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.
16
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 LTC3864 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.
Soft-start is enabled by connecting a capacitor from
the SS pin to ground. An internal 10µA current source
charges the capacitor, providing a linear ramping voltage
at the SS pin that causes V_OUT to rise smoothly from 0V
to its final regulated value. The total soft-start time will
be approximately:
t_SS
=
C_SS
•
0.8V
10µA
When the LTC3864 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the SS pin to scale the ramp rate appropriately. Two com-
mon implementations of tracking as shown in Figure 5a
are coincident and ratiometric. For coincident tracking,
make the divider ratio from the external supply the same
as the divider ratio for the feedback voltage. Ratiometric
tracking could be achieved by using a different ratio than
the feedback (Figure 5b).
Note that the soft-start capacitor charging current is always
flowing, producing a small offset error. To minimize this
error, select the tracking resistive divider values to be small
enough to make this offset error negligible.
Short-Circuit Faults: Current Limit and Foldback
The maximum inductor current is inherently limited in a
current mode controller by the maximum sense voltage.
In the LTC3864, the maximum sense voltage is 95mV,
measured across the inductor sense resistor R_SENSE,
placed across the V_IN and SENSE pins. The output current
limit is approximately:
I_LIMIT
≅
95mV
R_SENSE
–
∆I_L
2
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