LTC3811_15 Datasheet, PDF (27/48 Pages) Linear Technology – High Speed Dual, Multiphase Step-Down DC/DC Controller

LTC3811
APPLICATIONS INFORMATION
The value of the resistors in the RC filter is a tradeoff
between power dissipation and DC accuracy. The power
loss on R1 is:
( ) _{PR1 =}
V_IN – V_O
R1
• V_O
for a buck converter.
If the value of the filter resistor is too low, its power dis-
sipation will rise, resulting in a larger package size and
decreased efficiency at light load. If the value of the filter
resistor is too high, the input bias current flowing out of
the SENSE⁺ pin (approximately 1.5μA) could cause the
voltage drop across the resistor to be the same order
of magnitude of the peak sense voltage, which is also
undesirable. A good balance is to use a resistor value of
about 1k. An additional 1k resistor (R2) in the SENSE^– path
is used to compensate for the drop in the SENSE⁺ path,
and ideally these two resistors (R1 and R2) should match
one another.
In general, the larger the sense voltage range is, the smaller
the percentage error due to a mismatch in the filter resis-
tor IR drops. The current comparators were designed for
low offset and high speed, specifically for applications
requiring a small peak sense voltage.
Gate Drive Power Supply Considerations
The LTC3811 user has a choice of how to supply power to
the gate drivers and low voltage analog control circuitry.
The first of these is to use the internal low dropout linear
regulator, LDO, to draw power from V_IN and regulate DRV_CC
to 6V. The second way of supplying power to the gate
drivers and analog control circuitry is through the EXTV_CC
pin. The choice of which supply path to use depends upon
system flexibility, power dissipation and the maximum
junction temperature in the application.
The internal DRV_CC LDO is capable of sourcing up to
100mA, allowing the user to connect multiple power
MOSFETs in parallel on both channels for the high power
density applications. High input voltage applications in
which multiple large MOSFETs are being driven at high
frequencies, however, may cause the maximum junction
temperature rating for the LTC3811 to be exceeded.
In general, there are three potential sources of power
dissipation in the LTC3811:
1. The quiescent current consumed by all of the analog
control circuitry connected to INTV_CC
2. Gate drive losses
3. Losses in the LDO when power is being supplied from
V_IN
The steady-state quiescent current of the IC is typically
10mA and flows into the INTV_CC pin, either through the
LDO from V_IN or through an auxiliary power supply con-
nected to the EXTV_CC pin.
The second source of power dissipation is the gate drivers
connected to DRV_CC. The lower MOSFET gate drivers are
directly connected to DRV_CC and the upper ones are con-
nected to DRV_CC through the bootstrap diode and floating
supply capacitor C_B (refer to Functional Diagram). The gate
driver current requirement depends upon the number of
MOSFETs being driven, their total gate charge, Q_G(TOT),
and the operating frequency, f, of the converter. The total
current required by the low voltage circuitry is the sum of
the DC quiescent current and the gate drive current.
I_VCC = 10mA + Q_G(TOT) • f
If the internal LDO in the LTC3811 is used to supply power
to DRV_CC and INTV_CC, care should be taken to ensure that
the total low voltage current doesn’t exceed the 100mA
limit for the LDO.
Assuming that DRV_CC = EXTV_CC = INTV_CC = 6V, power
dissipation due to the quiescent current and gate drive
losses is:
PV_CC = 6V • (10mA + Q_G(TOT) • f)
The third source of power dissipation occurs in the LDO,
which supplies power to the DRV_CC pin when EXTV_CC is
less than 4.7V. When power is being drawn from V_IN the
power dissipated in the LDO is:
P_LDO = (V_IN – V_DRVCC) • (10mA + Q_G(TOT) • f)
3811f
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