FAN5037M Datasheet, PDF (7/11 Pages) Fairchild Semiconductor – Adjustable Switching Regulator Controller

PRODUCT SPECIFICATION
charged and discharged through the enabling and disabling
of the fixed current source. The variable current source is
controlled from the error inputs that are received from the
current and voltage feedback signals. The oscillator off-time
is controlled by the amount of current that is available from
FAN5037
the variable current source to charge the external capacitor up
to the high threshold level of the comparator. The on-time is
set by the constant current source that discharges the external
capacitor voltage down to the lower comparator threshold.
gm
CEXT 1
REF
gm
Constant On-Time Oscillator
IO
VH
VL
ION
6
GNDP
+5V
+12V
VCCA
2
4 IFBL
3 IFBH
8 VCCP
7 SDRV
5 FBSW
65-5037-07
VOUT
Figure 2. FAN5037 Detailed Block Diagram
Output Voltage Selection
The FAN5037 precision reference is trimmed to be 1.2V
nominally. When using the FAN5037, the system designer
has complete flexibility in choosing the output voltage for
one regulator from 1.2V to 3.6V. This is done by appropri-
ately selecting the feedback resistors. These could be 0.1%
resistors to realize optimum output accuracy. The following
equations determines the output voltage of the regulator:
V_OUT
=
1.2
×


-R----4--R--+---5--R-----5-_
(1)
For example, for 3.3V:
V_OUT
=
1.2
×


-R----4--R--+---5--R-----5-_
=
1.2 × _-3---.-4---8---2k---.-0-+--k---2---.-0----k-_
=
3.3V
Input Capacitors
The number of input capacitors required for the FAN5037 is
dependent on their ripple current rating, which assures their
rated life. The number required may be determined by
No. Caps = ^I---o--^u---^t--^*--------D-----C-----–-----D----C-- ²
(2)
I_rating
where the duty cycle DC = (Vout + Vf,diode) / Vin. For exam-
ple, with a 1.5V output at 10A, 5V input, and using the
Sanyo capacitors specified in Table 1 which have a 2A ripple
current rating, we have DC = (1.5 + .5)/5 = 0.4, and
No. Caps = ^1---0----*---------0---.-4-----–----0---.--4²= 2.44
2
so that we need 3 input capacitors.
Short Circuit Considerations
The FAN5037 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple current
flowing in the inductor. The FAN5037 will begin to limit the
output current to the load by reducing the duty cycle of the
top-side MOSFET driver when the voltage across the cur-
rent-sense resistor exceeds the short circuit comparator
threshold voltage (Vth). When this happens the output volt-
age will temporarily go out of regulation. As the voltage
across the sense resistor becomes larger, the duty cycle of the
top-side MOSFET will continue to be reduced until the cur-
rent limit value is reached. At this point, the FAN5037 will
continuously deliver the limit current at a reduced output
voltage level. The short circuit comparator threshold voltage
is typically 90mV, with a tolerance of ±10mV. The ripple
current flowing through the inductor in Figure 1 is 0.6Apeak.
Refer to Application Note AB-23 for detailed discussions.
The sense resistor value can be approximated as follows:
R_SENSE
=
V-----_{t-h---,-m---i--n-}
I_PK
×
(1
–
TF)
=
---------------V-----^t-^h--^,-^m----ⁱ-ⁿ----------------
0.6A + I_LOAD,MAX
×
(1
–
TF)
(3)
REV. 1.0.3 9/26/01
7