PRODUCT SPECIFICATION FAN5236
12 REV. 1.1.9 7/12/04
Current Processing Section
The following discussion refers to Figure 11.
The current through RSENSE resistor (ISNS) is sampled
shortly after Q2 is turned on. That current is held, and
summed with the output of the error amplifier. This effec-
tively creates a current mode control loop. The resistor con-
nected to ISNSx pin (RSENSE) 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 currrent
and line voltage. The following expression estimates the
recommended value of RSENSE as a function of the maxi-
mum load current (ILOAD(MAX)) and the value of the
MOSFET’s RDS(ON):
RSENSE must, however, be kept higher than:
Setting the Current Limit
A ratio of ISNS is also compared to the current established
when a 0.9 V internal reference drives the ILIM pin:
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 RSENSE is an
accurate representation of the load current. When using the
MOSFET as the sensing element, the variation of RDS(ON)
causes proportional variation in the ISNS. This value not
only varies from device to device, but also has a typical junc-
tion temperature coefficient of about 0.4% / °C (consult the
MOSFET datasheet for actual values), so the actual current
limit set point will decrease propotional to increasing
MOSFET die temperature. A factor of 1.6 in the current
limit setpoint should compensate for all MOSFET RDS(ON)
variations, assuming the MOSFET’s heat sinking will keep
its operating die temperature below 125°C.
Figure 12. Improving current sensing accuracy
More accurate sensing can be achieved by using a resistor
(R1) instead of the RDS(ON) of the FET as shown in Figure
12. This approach causes higher losses, but yields greater
accuracy in both VDROOP and ILIMIT . R1 is a low value
(e.g. 10mΩ) resistor.
Current limit (ILIMIT) should be set sufficiently high as to
allow inductor current to rise in response to an output load
transient. Typically, a factor of 1.2 is sufficient. In addition,
since ILIMIT is a peak current cut-off value, we will need to
multiply ILOAD(MAX) by the inductor ripple current (we’ll
use 25%). For example, in Figure 5 the target for ILIMIT
would be:
ILIMIT > 1.2 × 1.25 × 1.6 × 6A ≈14A (6)
Duty Cycle Clamp
During severe load increase, the error amplifier output can
go to its upper limit pushing a duty cycle to almost 100% for
significant amount of time. This could cause a large increase
of the inductor current and lead to a long recovery from a
transient, over-current condition, or even to a failure espe-
cially at high input voltages. To prevent this, the output of
the error amplifier is clamped to a fixed value after two clock
cycles if severe output voltage excursion is detected, limiting
the maximum duty cycle to
This circuit is designed to not interfere with normal PWM
operation. When FPWM is grounded, the duty cycle clamp
is disabled and the maximum duty cycle is 87%.
Gate Driver section
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. Also, it has functions that 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 adap-
tive dead-time circuit to work properly. Any delay along that
path will subtract from the delay generated by the adaptive
dead-time circit and shoot-through may occur.
RSENSE
ILOAD MAX()
RDS ON()
4.1K••
0.30 0.125 VIN MAX()
••
----------------------------------------------------------------------------- 1 0 0–= (4a)
RSENSE MIN()
ILOAD MAX()
RDS ON()
•
150µA
----------------------------------------------------------- 1 0 0–= (4b)
RILIM
11.2
ILIMIT
---------------- 100 RSENSE
+()
RDS ON()
----------------------------------------
×=(5)
LDRV
PGND
ISNS RSENSE
R1
Q2
DCMAX
VOUT
VIN
-------------- 2.4
VIN
---------+=