FAN5091MTC Fairchild Semiconductor, FAN5091MTC Datasheet - Page 15

• Manufacturer Part Number: FAN5091MTC
• Description: IC CTRLR DC-DC SYNC 2PH 24TSSOP
• Manufacturer: Fairchild Semiconductor

Specifications of FAN5091MTC

• Applications: Controller, Intel Pentium® IV
• Voltage - Input: 5V
• Number Of Outputs: 1
• Voltage - Output: 1.1 ~ 1.85 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP
• Output Voltage: 13.2 V
• Mounting Style: SMD/SMT
• Operating Temperature Range: 0 C to + 70 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PRODUCT SPECIFICATION
its on-time. Each slice has its own power ground pin, to per-
mit the slices to be placed in different locations without
affecting measurement accuracy. For best results, it is impor-
tant to connect the PGND and SW pins for each slice as a
Kelvin trace pair directly to the source and drain, respec-
tively, of the appropriate low-side MOSFET. Care is required
in the layout of these grounds; see the layout guidelines in
this datasheet.
Current Sharing
The two independent current sensors of the FAN5091 operate
with their independent current control loops to guarantee that
the two slices each deliver half of the total output current.
The only mismatch between the two slices occurs if there is a
mismatch between the R
Short Circuit Current Characteristics
The FAN5091 short circuit current characteristic includes a
function that protects the DC-DC converter from damage in
the event of a short circuit. The short circuit limit is set with
the R
with I
and R
Remember to make the R
effects of initial tolerance and temperature variation on the
MOSFETs’ R
tially above maximum operating current, to avoid nuisance
trips.
Important Note! The oscillator frequency must be selected
before selecting the current limit resistor, because the value
of RT is used in the calculation of R
When an overcurrent is detected, the high-side MOSFETs
are turned off, and the low-side MOSFETs are turned on,
and they remain in this state until the measured current
through the low-side MOSFET has returned to zero amps.
After reaching zero, the FAN5091 re-soft-starts, ensuring
that it can also safely turn on into a short.
A limitation on the current sense circuit is that I
must be less that 375mV. To ensure correct operation, use
I
will be some non-linearity in the short-circuit current not
accounted for in the equation.
As an example, consider the typical characteristic of the
DC-DC converter circuit with two FDP6670AL low-side
MOSFETs (R
= 7.8m each, or 3.9m total) in each slice, RT = 42.1K
(600KHz oscillator) and a 50K R
The converter exhibits a normal load regulation characteris-
tic until the voltage across the MOSFETs exceeds the inter-
nal short circuit threshold of 50K /(3.9m • 41.2K • 6.66)
REV. 1.0.0 5/10/01
SC
• R
S
SC
DS,on
resistor, as given by the formula
DS,on
the desired current limit, RT the oscillator resistor
R
one slice’s low-side MOSFET’s on resistance.
S
DS,on
DS
300mV; between 300mV and 375mV, there
= 6.5m maximum at 25 C • 1.2 at 75 C
. It is recommended to set I
=
I
SC
DS,on
S
large enough to include the
R
DS on
of the low-side MOSFETs.
S
S
.
.
RT 6.66
SC
SC
substan-
• R
DS,on
= 47A. [Note that this current limit level can be as high as
50K /(3.5m • 41.2K • 6.66) = 52A, if the MOSFETs
have typical R
At this point, the internal comparator trips and signals the
controller to leave on the low-side MOSFETs and keep off
the high-side MOSFETs. The inductor current decreases,
and power is not applied again until the inductor current
reaches 0A and the converter attempts to re-softstart.
Precision Current Sensing
The tolerances associated with the use of MOSFET current
sensing can be circumvented by the use of a current sense
resistor, as provided for by the FAN5092.
Light Load Efficiency
At light load, the FAN5091 uses a number of techniques to
improve efficiency. Because a synchronous buck converter is
two quadrant, able to both source and sink current, during
light load the inductor current will flow away from the out-
put and towards the input during a portion of the switching
cycle. This reverse current flow is detected by the FAN5091
as a positive voltage appearing on the low-side MOSFET
during its on-time. When reverse current flow is detected, the
low-side MOSFET is turned off for the rest of the cycle, and
the current instead flows through the body diode of the
high-side MOSFET, returning the power to the source. This
technique substantially enhances light load efficiency.
E*-mode
In addition, further enhancement in efficiency can be obtained
by putting the FAN5091 into E*-mode. When the Droop pin
is pulled to the 5V BYPASS voltage, the “A” slice of the
FAN5091 is completely turned off, reducing in half the
amount of gate charge power being consumed. E*-mode can
be implemented with the circuit shown in Figure 5:
Internal Voltage Reference
The reference included in the FAN5091 is a precision band-
gap voltage reference. Its internal resistors are precisely
trimmed to provide a near zero temperature coefficient (TC).
Based on the reference is the output from an integrated 5-bit
DAC. The DAC monitors the 5 voltage identification pins,
VID0-4, and scales the reference voltage from 1.100V to
1.850V in 25mV steps.
HI = E*-
mode on
Figure 5. Implementing E*-mode Control
DS,on
10K
rather than maximum, and are at 25 C.]
1K
2N2222
10K
BYPASS
2N2907
R
DROOP
FAN5091
pin 21
FAN5091
15