FAN5078 Datasheet, PDF (13/16 Pages) Fairchild Semiconductor – DDR/ACPI Regulator Combo

Current Processing Section
The following discussion refers to Figure 10.
The current through R_SENSE resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held and summed
with the output of the error amplifier. This effectively creates a
current mode control loop. R_SENSE sets the gain in the
current feedback loop. For stable operation, the voltage
induced by the current feedback at the PWM comparator input
should be set to 30% of the ramp amplitude at maximum load
current and line voltage.
Equation 5 estimates the recommended value of R_SENSE as
a function of the maximum load current ( I_LOAD(MAX) ) and the
value of the MOSFET’s R_DS(ON):
R_SENSE =
I_LOAD(MAX) • R_DS(ON) • 4.41K _{− 100}
30% • 0.125 • V_IN(MAX)
(5)
where R_DS(ON) is the maximum R_DS(ON) of the low-side
MOSFET at its maximum temperature.
R_SENSE must, however, be kept higher than:
R_SENSE(MIN) =
I_LOAD(MAX) • R_DS(ON)
145μA
− 100
(6)
Setting the Current Limit
ISNS is compared to the current established when a 0.9 V
internal reference drives the ILIM pin. R_ILIM, the R_DS(ON) of Q2,
and R_SENSE determine the current limit:
R_ILIM
=
9.6
I_LIMIT
X
(100 + R_SENSE )
R _DS(ON)
(7)
where ILIMIT is the peak inductor current. Since the tolerance
on the current limit is largely dependent on the ratio of the
external resistors it is fairly accurate if the voltage drop on the
switching node side of R_SENSE is an accurate representation
of the load current. When using the MOSFET as the sensing
element, the variation of R_DS(ON) causes proportional variation
in the ISNS. This value not only varies from device to device,
but also has a typical junction temperature coefficient of about
0.4% / °C (consult the MOSFET datasheet for actual values),
so the actual current limit set point decreases proportional to
increasing MOSFET die temperature. A factor of 1.6 in the
current limit set point should compensate for MOSFET R_DS(ON)
variations, assuming the MOSFET's heat sinking keeps its
operating die temperature below 125°C.
Current limit (I_LIMIT) should be set sufficiently high as to allow
the inductor current to rise in response to an output load
transient. Typically, a factor of 1.3 is sufficient. In addition,
since I_LIMIT is a peak current cut-off value, multiply I_LOAD(MAX)
by the inductor ripple current (i.e. 20%). To account all of
these variations, set ILIMIT as:
ILIMIT > I_LOAD(MAX) x 1.6 x 1.3 x 1.2
(8)
Q2
LDRV
ISNS ^RSENSE
PGND
Figure 11. Improving Current Sensing Accuracy
More accurate sensing can be achieved by using a resistor
(R1) instead of the R_DS(ON) of the FET, as shown in Figure
11. This approach causes higher losses, but greater accuracy.
Gate Drive
The adaptive gate control logic translates the internal PWM
control signal into the MOSFET gate drive signals, providing
necessary amplification, level shifting, and shoot-through
protection. It also has functions to help optimize the IC
performance over a wide range of operating conditions.
Since MOSFET switching time can vary dramatically from type
to type and with the input voltage, the gate control logic
provides adaptive dead time by monitoring the gate-to-source
voltages of both upper and lower MOSFETs. The lower
MOSFET drive is not turned on until the gate-to-source
voltage of the upper MOSFET has decreased to less than
approximately 1 volt. Similarly, the upper MOSFET is not
turned on until the gate-to-source voltage of the lower
MOSFET has decreased to less than approximately 1 volt.
This allows a wide variety of upper and lower MOSFETs to
be used without a concern for simultaneous conduction or
shoot-through.
There must be a low-resistance, low-inductance path between
the driver pin and the MOSFET gate for the adaptive dead-
time circuit to work properly. Any delay along that path
subtracts from the delay generated by the adaptive dead-time
circuit and shoot-through may occur.
© 2006 Fairchild Semiconductor Corporation
13
FAN5078 Rev. 1.0.0 • 05/11/06
www.fairchildsemi.com