LTC3863_15 Datasheet, PDF (17/38 Pages) Linear Technology – 60V Low IQ Inverting DC/DC Controller

LTC3863
Applications Information
The power MOSFET input capacitance, C_MILLER, is the most
important selection criteria for determining the transition
loss term in the P-channel MOSFET but is not directly speci-
fied on MOSFET data sheets. C_MILLER is a combination of
several components, but it can be derived from the typical
gate charge curve included on most data sheets (Figure 4).
The curve is generated by forcing a constant current out
of the gate of a common-source connected P-channel
MOSFET that is loaded with a resistor, and then plotting
the gate voltage versus time. The initial slope is the effect
of the gate-to-source and gate-to-drain capacitances. The
flat portion of the curve is the result of the Miller multipli-
cation effect of the drain-to-gate capacitance as the drain
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
MILLER EFFECT
V_SG
a
b
Q_IN
C_MILLER = (Q_B – Q_A)/V_SD(TEST)
(4a)
G
I_GATE
S
D
R_LOAD
+
– ^VSD(TEST)
3863 F04
(4b)
Figure 4. (4a) Typical P-Channel MOSFET Gate Charge
Characteristics and (4b) Test Set-Up to Generate Gate
Charge Curve
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