US3004,US3005
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
sel i t m axi um M O SFET Rds-
ecton, he
m
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)=2.1kΩ
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:
Note that since the MOSFETs Rds-on increases with
temperature, this number must be divided by
≈
1.5,
inorder to find the Rds-on max at room temperature. The
Motorola MTP3055VL has a maximum of 0.18Ω Rds-on
at room temperature, which meets our requirement.
To select the heatsink for the LDO Mosfet the first step
is to calculate the maximum power dissipation of the
device and then follow the same procedure as for the
switcher.
Pd = ( Vin - Vo ) * IL
Where :
Pd = Power Dissipation of the Linear Regulator
IL = Linear Regulator Load Current
For the 1.5V and 2A load:
Pd = (3.3 - 1.5)*2=3.6 W
Assuming Tj-max=125°C
Ts = Tj - Pd * (θjc +
θcs)
Ts = 125 - 3.6 * (1.8 + 0.05) = 118
°C
With the maximum heat sink temperature calculated in
the previous step, the Heat Sink to Air thermal resis-
tance (θsa) is calculated as follows :
Assuming Ta=35
°C
∆T
= Ts - Ta = 118 - 35 = 83
°C
Temperature Rise
Above Ambient
θsa
=
∆T/Pd
θsa
= 83 / 3.6 = 23
°C/W
The same heat sink as the one selected for the switcher
MOSFETs is also suitable for the 1.5V regulator. It is
also possible to use TO263 package or even the
MTD3055VL in D pak if the load current is less than
1.5A. For the 2.5V regulator since the dropout voltage is
only 0.8V and the load current is less than 0.5A, for
most applications the same MOSFET without heat sink
or for low cost applications, one can use
PN2222A in
TO92 or SOT23 package.
LDO Regulator Component Selection
35
×
10
−
5
.
F
SW
≈
C
T
Where
:
C
T
=Ti
min
g Capacitor
F
SW
=
Switching Frequency
If, F
SW
=
200
kHz :
C
T
≈
3.5
×
10
=
175
pF
200
×
10
3
−
5
LDO Power MOSFET Selection
The first step in selectiong the power MOSFET for the
linear regulators is to select its maximum Rds-on based
on the input to output Dropout voltage and the maximum
load current.
Rds(max)=(Vin - Vo)/IL
For Vo=1.5V, and Vin=3.3V , IL=2A
Rds-max=(3.3 - 1.5)/2= 0.9Ω
Since the internal voltage reference for the linear regula-
tors is set at 1.5V for all devices, there is no need to
divide the output voltage for the 1.5V, GTL+ regulator.
For the 2.5V, Clock supply the resistor dividers are se-
lected per following:
Vo=(1+Rt/Rb)*Vref
Where:
Rt=Top resistor divider
Rb=Bottom resistor divider
Vref=1.5V typical
Assuming Rt=100Ω, for Vo=2.5V
Rb=Rt/[(Vo/Vref) - 1]
Rb=100/[(2.5/1.5) - 1]=150Ω
4-12
Rev. 1.2
12/8/00
UNISEM [ UNISEM ]
