LTC3822-1_15 Datasheet, PDF (16/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous Step-Down DC/DC Controller

LTC3822-1
APPLICATIONS INFORMATION
Burst Mode Operation Considerations
The choice of R_DS(ON) and inductor value also determines
the load current at which the LTC3822-1 enters Burst Mode
operation. When bursting, the controller clamps the peak
inductor current to approximately:
I_BURST(PEAK)
=
1
4
•
∆V_SENSE(MAX)
R_DS(ON)
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f_OSC directly determine the
inductor’s peak-to-peak ripple current:
I_RIPPLE
=
V_OUT
V_IN
⎛ V_IN – V_OUT ⎞
⎝⎜ ^f_OSC ^{• L} ⎠⎟
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I_OUT(MAX). Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L ≥ ^{VIN – VOUT} • ^VOUT
f_OSC • I_RIPPLE V_IN
Inductor Core Selection
Once the inductance value is determined, the type of in-
ductor must be selected. Core loss is independent of core
size for a fixed inductor value, but it is very dependent
on inductance selected. As inductance increases, core
losses go down. Unfortunately, increased inductance
requires more turns of wire and therefore copper losses
will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design current
is exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Schottky Diode Selection (Optional)
The Schottky diode D in Figure 10 conducts current dur-
ing the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
N-channel MOSFET from turning on and storing charge
during the dead time, which could cost as much as 1%
in efficiency. A 1A Schottky diode is generally a good size
for most applications, since it conducts a relatively small
average current. Larger diodes result in additional transition
losses due to their larger junction capacitance. This diode
may be omitted if the efficiency loss can be tolerated.
C_IN and C_OUT Selection
In continuous mode, the source current of the top MOSFET
is a square wave of duty cycle (V_OUT/V_IN). To prevent large
voltage transients, a low ESR input capacitor sized for the
maximum RMS current must be used. The maximum RMS
capacitor current is given by:
( ) _{CIN Required IRMS}
≈ I_MAX
• ^VOUT
•
V_IN – V_OUT
V_IN
1/2
This formula has a maximum value at V_IN = 2V_OUT, where
I_RMS = I_OUT/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of life.
This makes it advisable to further derate the capacitor or
to choose a capacitor rated at a higher temperature than
required. Several capacitors may be paralleled to meet the
size or height requirements in the design. Due to the high
operating frequency of the LTC3822-1, ceramic capacitors
can also be used for C_IN. Always consult the manufacturer
if there is any question.
38221f
16