US3007
External Shutdown
R19= 100*{Vdac /(Vo - 1.004*Vdac)} [W]
R19= 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.
The best way to shutdown the US3007 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.
3.3V supply
The loop gain for the non synchronous switching regula-
tor is intentionally set low to take advantage of the level
shifting technique to reduce the number of output ca-
pacitors. Typically there is a 1% drop in the output volt-
age from light load (discontinous conduction mode) to
full load (continous conduction mode) in the 3.3V sup-
ply. To account for this, the output voltage is set at 3.5V
typically. The same procedure as for the synchronous is
applied to the non synch with the exception that the
internal voltage reference of this regulator is internally
set at 2V. The following is the set of equations to use for
the output voltage setting for the non-synchronous as-
suming the Vo=3.5V and the top resistor,( R2 in the ap-
plication circuit ) is; R2=75W. The bottom resistor, R3 is
calculated as follows:
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 capacitors C3 and C14 and the high
side mosfets ,Q1 and Q3 as close to their respective
input caps as possible
R3= R2*{2 /(Vo - 2)} [W]
R3= 75*{2 /(3.5 - 2)} = 100W , 1%
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. Repeat this for the Q1 and D1 for the non syn-
chronous.
Note: The value of the top resistor , R2 must not
exceed 100W.
Soft Start Capacitor Selection
3) Place the snubber R15 & C13 between Q4 & Q3.
Repeat this for R1 and C4 with respect to the Q1 and D1
for the non synchronous.
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.
Repeat this for L2 with respect to the C1 for the non
synchronous.
5) Place the bypass capacitors, C8 and C19 right next
to 12V and 5V pins. C8 next to the 12V, pin 28 and C19
next to the 5V, pin 11.
6) Place the US3007 such that the pwm output drives,
pins 27 and 25 are relatively short distance from gates
of Q3 and Q4. The non-synch MOSFET must also be
situated such that the distance from its gate to the pin 1
of the US3007 is also relatively short.
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:
I=9000uF*(1V/100mS)=0.09A
Input Filter
7) Place all resistor dividers close to their respective
feedback pins.
8) Place the 2.5V output capacitor, C18 close to the
pin16 of the IC and the 1.5V output capacitor, C17 close
to the Q2 MOSFET.
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.
Rev. 1.8
12/8/00
4-14
UNISEM [ UNISEM ]
