LTC3814-5_15 Datasheet, PDF (12/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
APPLICATIONS INFORMATION
Power MOSFET Selection
The LTC3814-5 requires two external N-channel power
MOSFETs, one for the bottom (main) switch and one for
the top (synchronous) switch. Important parameters for
the power MOSFETs are the breakdown voltage BV_DSS,
threshold voltage V_(GS)TH, on-resistance R_DS(ON), Miller
capacitance and maximum current I_DS(MAX).
Since the bottom MOSFET is used as the current sense
element, particular attention must be paid to its on-resis-
tance. MOSFET on-resistance is typically specified with
a maximum value R_DS(ON)(MAX) at 25°C. In this case,
additional margin is required to accommodate the rise in
MOSFET on-resistance with temperature:
R_DS(ON)(MAX
)
=
R_SENSE
ρ_T
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature (see Figure 2) and typically varies
from 0.4%/°C to 1.0%/°C depending on the particular
MOSFET used.
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
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Figure 2. R_DS(ON) vs Temperature
The most important parameter in high voltage applications
is breakdown voltage BV_DSS. Both the top and bottom
MOSFETs will see full output voltage plus any additional
ringing on the switch node across its drain-to-source dur-
ing its off-time and must be chosen with the appropriate
breakdown specification. The LTC3814-5 is designed to
be used with a 4.5V to 14V gate drive supply (INTV_CC pin)
for driving logic-level MOSFETs (V_GS(MIN) ≥ 4.5V).
For maximum efficiency, on-resistance R_DS(ON) and input
capacitance should be minimized. Low R_DS(ON) minimizes
conduction losses and low input capacitance minimizes
transition losses. MOSFET input capacitance is a combi-
nation of several components but can be taken from the
typical “gate charge” curve included on most data sheets
(Figure 3).
V_OUT
MILLER EFFECT
V_GS
V
a
b
Q_IN
C_MILLER = (Q_B – Q_A)/V_DS
+
+
V_GS
V_DS
–
–
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Figure 3. Gate Charge Characteristic
The curve is generated by forcing a constant input cur-
rent into the gate of a common source, current source
loaded stage and then plotting the gate voltage versus
time. The initial slope is the effect of the gate-to-source
and the gate-to-drain capacitance. The flat portion of the
curve is the result of the Miller multiplication effect of the
drain-to-gate capacitance as the drain drops the voltage
across the current source load. The upper sloping line is
due to the drain-to-gate accumulation capacitance and
the gate-to-source capacitance. 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_DS drain
voltage, but can be adjusted for different V_DS voltages by
multiplying by the ratio of the application V_DS to the curve
specified V_DS values. A way to estimate the C_MILLER term
is to take the change in gate charge from points a and b
on a manufacturers data sheet and divide by the stated
V_DS voltage specified. C_MILLER is the most important se-
lection criteria for determining the transition loss term in
the top MOSFET but is not directly specified on MOSFET
data sheets. C_RSS and C_OS are specified sometimes but
definitions of these parameters are not included.
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