LTC3832 Datasheet, PDF (15/24 Pages) Linear Technology – High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation

LTC3832/LTC3832-1
APPLICATIO S I FOR ATIO
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 = LTC3832 oscillator frequency = 300kHz
L_O = Inductor value
Solving this equation with our typical 3.3V to 2.5V appli-
cation with a 1µH inductor, we get:
(3.3V – 2.5V) •
300kHz • 1µH •
2.5V
3.3V
=
2A_P-P
Peak inductor current at 10A load:
10A + (2A/2) = 11A
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 LTC3832 design places significant demands on
both the input and the output capacitors. During normal
steady load operation, a buck converter like the LTC3832
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
LTC3832 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. An 5A load step with a 0.05Ω ESR output
capacitor results in a 250mV output voltage shift; this is
10% of the output voltage for a 2.5V 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 rated for use in switching power
supplies with specified ripple current ratings and ESR can
be used effectively in LTC3832 applications. OS-CON
electrolytic capacitors from Sanyo and other manufactur-
ers give excellent performance and have a very high
performance/size ratio for electrolytic capacitors. Surface
mount applications can use either electrolytic or dry
tantalum capacitors. Tantalum capacitors must be surge
tested and specified for use in switching power supplies.
Low cost, generic tantalums are known to have very short
lives followed by explosive deaths in switching power
supply applications. Other capacitors that can be used
include the Sanyo POSCAP and MV-WX series.
A common way to lower ESR and raise ripple current
capability is to parallel several capacitors. A typical
sn3832 3832fs
15