LTC3802 Datasheet, PDF (22/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
on-resistance. MOSFET on-resistance is typically speci-
fied 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 due to self heating and
higher ambient temperature:
R_DS(ON)(MAX) (T) = ρ_T • R_DS(ON)(MAX) (25°C)
The ρ_T term is a normalization factor (unity at 25°C)
accounting for the significant variation in on-resistance
with temperature, typically about 0.4%/°C as shown in
Figure 8a. For a maximum junction temperature of 100°C,
using a value ρ_T = 1.3 is reasonable.
MOSFET input capacitance is a combination of several
components but can be taken from the typical “gate
charge” curve included on most data sheets (Figure 8b).
The curve is generated by forcing a constant input current
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
2.0
1.5
1.0
0.5
0
–50
0
50
100
150
JUNCTION TEMPERATURE (°C)
3802 F08a
Figure 8a. Typical MOSFET R_DS(ON) vs Temperature
V_IN
MILLER EFFECT
V_GS
V
a
b
Q_IN
C_MILLER = (Q_B – Q_A)/V_DS
+
+
V_GS
V_DS
–
–
3802 F08b
Figure 8b. Gate Charge Characteristics
22
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 manufac-
turers data sheet and divide by the stated V_DS voltage
specified. C_MILLER is the most important selection 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.
When the controller is operating in continuous mode the
duty cycles for the top and bottom MOSFETs are given by:
Top Gate Duty Cycle = ^VOUT
V_IN
Bottom
Gate
Duty
Cycle
=


V_IN
– V_OUT
V_IN


The power dissipation for the top and bottom MOSFETs at
maximum output current are given by:
( )^{( )( ) P}TOP
=
V_OUT
V_IN
I_OUT(MAX)2
ρ_T(TOP)
R_DS(ON)(MAX)
( )( ) ^+V_IN2


I_OUT(MAX)
2


R_DR
C _MILLER
•

 ^PVCC
1
– V_TH(IL)
+
1^
V_{TH(IL) }
• f_sw
( )^{( )( ) P}BOT
=
V_IN
– V_OUT
V_IN
I_OUT(MAX)2
ρ_T(TOP)
R_DS(ON)(MAX)
where:
R_DR = Effective top driver resistance
V_TH(IL) = MOSFET data sheet specified typical gate
threshold voltage at the specified drain current
3802f