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US3018CW

5 BIT PROGRAMMABLE SYNCHRONOUS BUCK PLUS LDO CONTROLLER AND 200mA LDO ON BOARD

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US3018

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

Slot 1 and back to the GND pin of the 3018 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(R17 in the applica-

tion circuit) is set at 100Ω, and the R19 is calculated.

For example, if DAC voltage setting is for 2.8V and the

desired output under light load is 2.835V, then R19 is

calculated using the following formula :

R19= 100*{Vdac /(Vo - 1.004*Vdac)} [Ω]

R19= 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

limit treshold. A minimum of 1uF capacitor insures this

for most applications. An internal resistor charges the

soft start capacitor which slowly ramps up the inverting

cap thereby limiting the input current. For example, with

1uF of soft start capacitor, the ramp up rate is approxi-

mated to be 1V/20mS. For example if the output ca-

pacitance is 9000uF, the maximum start up current will

be:

I=9000uF*(1V/20mS)=0.45A

The other function of the soft start cap is to provide an off

time between the current limit cycles(HICCUP) in order

for the synchronous MOSFET to cool off and survive the

short circuit condition. The off time between the current

limit cycles is appoximated as:

T(hicup)=60*Css (mS)

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.

External Shutdown

The best way to shutdown the US3018 is to pull down

on the soft start pin using an external small signal tran-

sistor such as 2N3904 or 2N7002 small signal MOSFET.

This allows slow ramp up of the output, the same as the

power up.

Layout Considerations

Switching regulators require careful attention to the lay-

out of the components, specifically power components

since they switch large currents. These switching com-

ponents can create large amount of voltage spikes and

high frequency harmonics if some of the critical compo-

nents are far away from each other and are connected

with inductive traces. The following is a guideline of how

to place the critical components and the connections

between them in order to minimize the above issues.

Start the layout by first placing the power components:

1) Place the input capacitor C14 and the high side

mosfet, Q3 as close to each other as possible.

2) Place the synchronous mosfet,Q4 and the Q3 as

close to each other as possible with the intention that

the source of Q3 and drain of the Q4 has the shortest

length.

3) Place the snubber R15 & C13 between Q4 & Q3.

4) Place the output inductor ,L3 and the output capaci-

tors ,C16 between the mosfet and the load with output

capacitors distributed along the slot 1 and close to it.

5) Place the bypass capacitors, C8 and C19 right next

to 12V and 5V pins. C8 next to the 12V, pin 1 and C19

next to the 5V, pin 8.

6) Place the US3018 such that the pwm output drives,

pins 24 and 22 are relatively short distance from gates

of Q3 and Q4.

7) Place all resistor dividers close to their respective

feedback pins.

4-14

Rev. 1.4

12/8/00

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