LTC3772B Datasheet, PDF (12/20 Pages) Linear Technology – Micropower No RSENSE Constant Frequency Step-Down DC/DC Controller

LTC3772B
APPLICATIO S I FOR ATIO
Under normal load conditions, the average current con-
ducted by the diode is:
I_D
=
⎛
⎝⎜
V_IN − V_OUT
V_IN + V_D
⎞
⎠⎟
I_OUT
The allowable forward voltage drop in the diode is calcu-
lated from the maximum short-circuit current as:
V_F
≅
P_D
I_PEAK
where P_D is the allowable power dissipation and will be
determined by efficiency and/or thermal requirements.
A fast switching diode must also be used to optimize effi-
ciency. Schottky diodes are a good choice for low forward
drop and fast switching times. Remember to keep lead
length short and observe proper grounding to avoid ring-
ing and increased dissipation.
An additional consideration in applications where low no-
load quiescent current is critical is the reverse leakage
current of the diode at the regulated output voltage. A leak-
age greater than several microamperes can represent a
significant percentage of the total input current.
C_IN and C_OUT Selection
The input capacitance, C_IN, is needed to filter the trapezoi-
dal current at the source of the top MOSFET. To prevent large
ripple voltage, a low ESR input capacitor sized for the
maximum RMS current should be used. RMS current is
given by:
I_RMS
=
I_OUT(MAX)
V_OUT
V_IN
V_{IN – 1}
V_OUT
This formula has a maximum at V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that ripple current ratings from
capacitor manufacturers are often based on only 2000 hours
of life which makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
size or height requirements in the design.
12
The output filtering capacitor C smooths out current flow
from the inductor to the load, help maintain a steady out-
put voltage during transient load changes and reduce output
voltage ripple. The capacitors must be selected with suf-
ficiently low ESR to minimize voltage ripple and load step
transients and sufficiently bulk capacitance to ensure the
control loop stability.
The output ripple, ∆V_OUT, is determined by:
∆V_OUT
≤
∆I_L
⎛
⎝⎜
ESR
+
1
8fC _OUT
⎞
⎠⎟
The output ripple is highest at maximum input voltage since
∆I_L increases with input voltage. Multiple capacitors placed
in parallel may be needed to meet the ESR and RMS cur-
rent handling requirements. Dry tantalum, special polymer,
aluminum electrolytic and ceramic capacitors are all avail-
able in surface mount packages. Special polymer capaci-
tors offer very low ESR but have lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density but it is important to only use types
that have been surge tested for use in switching power sup-
plies. Aluminum electrolytic capacitors have significantly
higher ESR but can be used in cost-sensitive applications
provided that consideration is given to ripple current rat-
ings 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.
Using Ceramic Input and Output Capacitors
Higher values, lower cost ceramic capacitors are now
becoming available in smaller case sizes. Their high ripple
current, high voltage rating and low ESR make them ideal
for switching regulator applications. However, care must
be taken when these capacitors are used at the input and
output. When a ceramic capacitor is used at the input and
the power is supplied by a wall adapter through long wires,
a load step at the output can induce ringing at the input, V_IN.
At best, this ringing can couple to the output and be mis-
taken as loop instability. At worst, a sudden inrush of cur-
rent through the long wires can potentially cause a voltage
spike at V_IN large enough to damage the part.
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