US3012/3012A
Following the same procedure for the Schottcky diode
results in a heatsink with qsa = 25 °C/W. Although it is
possible to select a slightly smaller heatsink, for sim-
plicity the same heatsink as the one for the high side
MOSFET is also selected for the synchronous MOSFET.
Slot 1 and back to the GND pin of the device is 5mW and
if the total DI, the change from light load to full load is
14A, then the output voltage measured at the top of the
resistor divider which is also connected to the output
capacitors in this case, must be set at half of the 70 mV
or 35mV higher than the DAC voltage setting. To do this,
the top resistor of the resistor divider(R10 in the applica-
tion circuit) is set at 100W, and the R11 is calculated.
For example, if DAC voltage setting is for 2.8V and the
desired output under light load is 2.835V, then R11 is
calculated using the following formula :
R11= 100*{Vdac /(Vo - 1.004*Vdac)} [W]
R11= 100*{2.8 /(2.835 - 1.004*2.800)} = 11.76 kW
Select 11.8 kW , 1%
Note: The value of the top resistor must not exceed
100W. The bottom resistor can then be adjusted to raise
the output voltage.
Switcher Current Limit Protection
The PWM controller uses the MOSFET Rds-on as the
sensing resistor to sense the MOSFET current and com-
pares to a programmed voltage which is set externally
via a resistor (Rcs) placed between the drain of the
MOSFET and the “CS+” terminal of the IC as shown in
the application circuit. For example, if the desired cur-
rent limit point is set to be 22A and from our previous
selection,themaximum MOSFETRds-on=19mW, then
the current sense resistor, Rcs is calculated as :
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1kW
Soft Start Capacitor Selection
Where: Ib=200uA is the internal current setting of the
device
The soft start capacitor must be selected such that dur-
ing the start up when the output capacitors are charging
up, the peak inductor current does not reach the current
limit treshold. A minimum of 1uF capacitor insures this
for most applications. An internal 10uA current source
charges the soft start capacitor which slowly ramps up
the inverting input of the PWM comparator Vfb3. This
insures the output voltage to ramp at the same rate as
the soft start cap thereby limiting the input current. For
example, with 1uF and the 10uA internal current source
the ramp up rate is (DV/ Dt)=I/C = 1V/100mS. Assum-
ing that the output capacitance is 9000uF, the maxi-
mum start up current will be:
Switcher Timing Capacitor Selection
The switching frequency can be programmed using an
external timing capacitor. The Ct value can be approxi-
mated using the equation below:
3.5´ 10- 5
FSW
»
C
T
Where :
CT
=Ti min g Capacitor
I=9000uF*(1V/100mS)=0.09A
SW
F
=
Switching Frequency
Input Filter
It is highly recommended to place an inductor between
the system 5V supply and the input capacitors of the
switching regulator to isolate the 5V supply from the
switching noise that occurs during the turn on and off of
the switching components. Typically an inductor in the
range of 1 to 3 uH will be sufficient in this type of appli-
cation.
If, FSW=200 kHz :
3.5´ 10- 5
CT
»
= 175pF
200 ´ 103
Switcher Output Voltage Adjust
As it was discussed earlier,the trace resistance from
the output of the switching regulator to the Slot 1 can be
used to the circuit advantage and possibly reduce the
number of output capacitors, by level shifting the DC
regulation point when transitioninig from light load to full
load and vice versa. To account for the DC drop, the
output of the regulator is typically set about half the DC
drop that results from light load to full load. For example,
if the total resistance from the output capacitors to the
Switcher External Shutdown
The best way to shutdown the part is to pull down on the
soft start pin using an external small signal transistor
such as 2N3904 or 2N7002 small signal MOSFET. This
allows slow ramp up of the output, the same as the power
up.
Rev. 1.0
5/6/98
4-10
UNISEM [ UNISEM ]
