LTC3863 Datasheet, PDF (17/36 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller Wide Operating VIN Range: 3.5V to 60V

LTC3863
APPLICATIONS INFORMATION
voltage rises across the resistor load. The Miller charge
(the increase in coulombs on the horizontal axis from a to
b while the curve is flat) is specified for a given V_SD test
voltage, but can be adjusted for different V_SD voltages by
multiplying by the ratio of the adjusted V_SD to the curve
specified V_SD value. A way to estimate the C_MILLER term
is to take the change in gate charge from points a and b
(or the parameter Q_GD on a manufacturer’s data sheet)
and dividing it by the specified V_SD test voltage, V_SD(TEST).
C_MILLER
≅
Q_GD
V_{SD( TEST )}
The term with C_MILLER accounts for transition loss, which
is highest at high input voltages. For V_IN < 20V, the high
current efficiency generally improves with larger MOSFETs,
while for V_IN > 20V, the transition losses rapidly increase
to the point that the use of a higher R_DS(ON) device with
lower C_MILLER actually provides higher efficiency.
Schottky Diode Selection
When the P-channel MOSFET is turned off, a power
Schottky diode is required to function as a commutating
diode to carry the inductor current. The average forward
diode current is independent of duty factor and is de-
scribed as:
I_F(AVG) = I_OUT
The worst-case condition for diode conduction is a short-
circuit condition where the Schottky must handle the
maximum current as its duty factor approaches 100% (and
the P-channel MOSFET’s duty factor approaches 0%). The
diode therefore must be chosen carefully to meet worst-
case voltage and current requirements. A good practice
is to choose a diode that has a forward current rating two
times higher than I_OUT(MAX).
Once the average forward diode current is calculated, the
power dissipation can be determined. Refer to the Schottky
diode data sheet for the power dissipation, P_DIODE, as a
function of average forward current, I_F(AVG). P_DIODE can
also be iteratively determined by the two equations below,
where V_F(IOUT,_TJ) is a function of both I_F(AVG) and junction
temperature T_J. Note that the thermal resistance, θ_JA(DIODE),
given in the data sheet is typical and can be highly layout
dependent. It is therefore important to make sure that the
Schottky diode has adequate heat sinking.
T_J ≅ P_DIODE • θ_JA(DIODE)
P_DIODE ≅ I_F(AVG) • V_D(IOUT,TJ)
The Schottky diode forward voltage is a function of both
I_F(AVG) and T_J, so several iterations may be required to
satisfy both equations. The Schottky forward voltage,
V_D, should be taken from the Schottky diode data sheet
curve showing instantaneous forward voltage. The forward
voltage will change as a function of both T_J and I_F(AVG).
The nominal forward voltage will also tend to increase as
the reverse breakdown voltage increases. It is therefore
advantageous to select a Schottky diode appropriate to the
input voltage requirements. The diode reverse breakdown
voltage must meet the following condition:
V_R > V_IN(MAX) + |V_OUT|
C_IN and C_OUT Selection
The input and output capacitance, C_IN/C_OUT, are required
to filter the square wave current through the P-channel
MOSFET and diode respectively. Use a low ESR capacitor
sized to handle the maximum RMS current:
I_CIN(RMS) = I_COUT(RMS) = I_OUT •
| V_OUT | +V_D
V_IN
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3863f
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