LTC3831 Datasheet, PDF (15/20 Pages) Linear Technology – High Power Synchronous Switching Regulator Controller for DDR Memory Termination

LTC3831
APPLICATIO S I FOR ATIO
time in this application, resulting in a 8.8µs delay in
responding to a 5A load current step. During this 8.8µs,
the difference between the inductor current and the output
current is made up by the output capacitor. This action
causes a temporary voltage droop at the output. To
minimize this effect, the inductor value should usually be
in the 1µH to 5µH range for most LTC3831 circuits. To
optimize performance, different combinations of input
and output voltages and expected loads may require
different inductor values.
Once the required value is known, the inductor core type
can be chosen based on peak current and efficiency
requirements. Peak current in the inductor will be equal to
the maximum output load current plus half of the peak-to-
peak inductor ripple current. Ripple current is set by the
inductor value, the input and output voltage and the
operating frequency. The ripple current is approximately
equal to:
I_RIPPLE
=
(V_IN − V_OUT)
f_OSC • L_O
• (V_OUT)
• V_IN
f_OSC = LTC3831 oscillator frequency = 200kHz
L_O = Inductor value
Solving this equation with our typical 2.5V to 1.25V
application with 2µH inductor, we get:
(2.5V – 1.25V) • 1.25V
200kHz • 2µH • 2.5V
=
1.56A_P-P
Peak inductor current at 5A load:
5A + (1.56A/2) = 5.78A
The ripple current should generally be between 10% and
40% of the output current. The inductor must be able to
withstand this peak current without saturating, and the
copper resistance in the winding should be kept as low as
possible to minimize resistive power loss. Note that in
circuits not employing the current limit function, the
current in the inductor may rise above this maximum
under short circuit or fault conditions; the inductor should
be sized accordingly to withstand this additional current.
Inductors with gradual saturation characteristics are often
the best choice.
Input and Output Capacitors
A typical LTC3831 design places significant demands on
both the input and the output capacitors. During normal
steady load operation, a buck converter like the LTC3831
draws square waves of current from the input supply at the
switching frequency. The peak current value is equal to the
output load current plus 1/2 the peak-to-peak ripple cur-
rent. Most of this current is supplied by the input bypass
capacitor. The resulting RMS current flow in the input
capacitor heats it and causes premature capacitor failure
in extreme cases. Maximum RMS current occurs with
50% PWM duty cycle, giving an RMS current value equal
to I_OUT/2. A low ESR input capacitor with an adequate
ripple current rating must be used to ensure reliable
operation. Note that capacitor manufacturers’ ripple cur-
rent ratings are often based on only 2000 hours (3 months)
lifetime at rated temperature. Further derating of the input
capacitor ripple current beyond the manufacturer’s speci-
fication is recommended to extend the useful life of the
circuit. Lower operating temperature has the largest effect
on capacitor longevity.
The output capacitor in a buck converter under steady-
state conditions sees much less ripple current than the
input capacitor. Peak-to-peak current is equal to inductor
ripple current, usually 10% to 40% of the total load
current. Output capacitor duty places a premium not on
power dissipation but on ESR. During an output load
transient, the output capacitor must supply all of the
additional load current demanded by the load until the
LTC3831 adjusts the inductor current to the new value.
ESR in the output capacitor results in a step in the output
voltage equal to the ESR value multiplied by the change in
load current. A 5A load step with a 0.05Ω ESR output
capacitor results in a 250mV output voltage shift; this is
20% of the output voltage for a 1.25V supply! Because of
the strong relationship between output capacitor ESR and
output load transient response, choose the output capaci-
tor for ESR, not for capacitance value. A capacitor with
suitable ESR will usually have a larger capacitance value
than is needed to control steady-state output ripple.
Electrolytic capacitors, such as the Sanyo MV-WX series,
rated for use in switching power supplies with specified
ripple current ratings and ESR, can be used effectively in
3831f
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