US3010 Datasheet, PDF (10/11 Pages) UNISEM – 5 BIT PROGRAMMABLE SYNCHRONOUS BUCK CONTROLLER IC

US3010/3010A
Following the same procedure for the Schottcky diode
results in a heatsink with θsa = 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.
Switcher Current Limit Protection
The IC uses the MOSFET Rds-on as the sensing resis-
tor to sense the MOSFET current and compares to a
programmed voltage which is set externally via a resis-
tor (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 current limit point is
set to be 22A and from our previous selection, the maxi-
mum MOSFET Rds-on=19mΩ, then the current sense
resistor, Rcs is calculated as :
Vcs=IcL*Rds=22*0.019=0.418V
Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1kΩ
Where: Ib=200uA is the internal current setting of the
device
Switcher Timing Capacitor Selection
The US3010A frequency can be programmed using an
external timing capacitor. The value of Ct can be ap-
proximated using the equation below:
FSW ≈ ^{3.5 × 10−5}
CT
Where :
CT=Ti min g Capacitor
FSW = Switching Frequency
Slot 1 and back to the GND pin of the device is 5mΩ and
if the total ∆I, 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, Rtop is set at 100Ω,
and the bottom resistor, Rb is calculated. For example,
if DAC voltage setting is for 2.8V and the desired output
under light load is 2.835V, then Rb is calculated using
the following formula :
Rb= 100*{Vdac /(Vo - 1.004*Vdac)} [Ω]
Rb= 100*{2.8 /(2.835 - 1.004*2.800)} = 11.76 kΩ
Select 11.8 kΩ , 1%
Note: The value of the top resistor must not exceed
100Ω. The bottom resistor can then be adjusted to raise
the output voltage.
Soft Start Capacitor Selection
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 (∆V/ ∆t)=I/C = 1V/100mS. Assum-
ing that the output capacitance is 9000uF, the maxi-
mum start up current will be:
I=9000uF*(1V/100mS)=0.09A
Input Filter
If, FSW=200 kHz :
CT
≈
3.5 × 10^−5
200 × 10³
= 175pF
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
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.
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.
4-10
Rev. 1.1
5/18/98