4-10

Rev. 1.3

12/8/00

US3011

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 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, the maximum MOSFET Rds-on=19m

Ω, then

the current sense resistor, Rcs is calculated as :

Vcs=IcL*Rds=22*0.019=0.418V

Rcs=Vcs/Ib=(0.418V)/(200uA)=2.1k

Ω

Where: Ib=200uA is the internal current setting of the

device

Switcher Timing Capacitor Selection

The switching frequency can be programmed using an

external timing capacitor. The value of Ct can be ap-

proximated using the equation below:

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

F

C

Where

C =Ti

g Capacitor

F

Switching Frequency

SW

T

T

SW

≈

×

=

−

3 5 10

5

.

:

min

If, F =

kHz :

C

pF

SW

T

200

3 5 10

200 10

175

5

3

≈

×

×

=

−

.

Slot 1 and back to the GND pin of the device is 5m

Ω 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 100

Ω,

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 k

Ω , 1%

Note: The value of the top resistor must not exceed

100

Ω

Ω. 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

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.