SP6122 Datasheet, PDF (14/19 Pages) Sipex Corporation – Low Voltage, Micro 8, PFET, Buck Controller Ideal for 1A to 5A, Small Footprint, DC-DC Power Converters

P_SH(MAX) ≅ 1/2 I_OUT(MAX)V_IN(MAX)(t_{RISE +} t_FALL)F_S
where t_RISE (SP6122) for 8A PMOS is typi-
cally 20ns and t_FALL (SP6122) for 8A PMOS
is typically 40ns.
Switching losses need to be taken into
account for high switching frequency, since
they are directly proportional to switching
frequency. The conduction losses associ-
ated with the PMOS is determined by:
P_CH(MAX) = I_{OUT (MAX)} ² R_DS(ON) D
Where R_DS(ON) = drain to source on resis-
tance.
The total power losses of the PMOS are the
sum of switching and conduction losses.
For input voltages of 3.3V and 5V, conduc-
tion losses often dominate switching losses.
Therefore, lowering the R_DS(ON) of the PMOS
always improves efficiency even though it
gives rise to higher switching losses due to
increased C_RSS.
For the SP6122 design example, the
Fairchild PMOS PDS6375 was selected for
its low R_DS(ON) and good switching charac-
teristics including low gate charge at the
3.3V input. Using table 1 values for R_DS(ON)
and t_RISE and t_FALL for the SP6122, we
calculate;
P_SH(MAX) = 119mW and P_CH(MAX) = 203mW.
R_DS(ON) varies greatly with the gate driver
voltage. The MOSFET vendors often specify
R_DS(ON) on multiple gate to source voltages
(V_GS), as well as provide typical curve of
R_DS(ON) versus V_GS. For 5V input, use the
R_DS(ON) specified at 4.5V V_GS. At the time of
this publication, vendors, such as Fairchild,
Siliconix and International Rectifier, have
started to specify R_DS(ON) at V_GS less than
3V. This has provided necessary data for
designs in which these MOSFETs are driven
with 3.3V and made it possible to use
SP6122 in 3.3V only applications.
APPLICATION INFORMATION: Continued
Thermal calculation must be conducted to
ensure the MOSFET can handle the maxi-
mum load current. The junction tempera-
ture of the MOSFET, determined as follows,
must stay below the maximum rating.
T_J(MAX) = T_{A (MAX)} + P_MOSFET(MAX) Rθ_JA
where
T_{A (MAX)} = maximum ambient temperature
P_MOSFET(MAX) = maximum power dissipation
of the MOSFET, including both switching
and conduction losses
Rθ_JA = junction to ambient thermal resistance.
Rθ_JA of the device depends greatly on the
board layout, as well as device package.
Significant thermal improvement can be
achieved in the maximum power dissipation
through the proper design of copper mount-
ing pads on the circuit board. For example,
in a SO-8 package, placing two 0.04 square
inches copper pad directly under the pack-
age, without occupying additional board
space, can increase the maximum power
from approximately 1 to 1.2W.
For the PMOS PDS6375, assuming T_{A (MAX)}
= 20°C, P_MOSFET(MAX) = P_SH(MAX) + P_CH(MAX)
= 321mW, and assuming per PDS6375
data sheet, Rθ_JA = 50°C/W if using 0.5 in²
pad of 2oz Cu,
T_J(MAX) = 36°C
which is only a 16°C rise from ambient.
SCHOTTKY DIODE SELECTION
The schottky diode is selected for low for-
ward voltage, current capability and fast
switching speed. The average power dissi-
pation of the schottky diode is determined
by
P_DIODE = V_F I_OUT (1- D)
Where V_F is the forward voltage of the
Schottky diode at I_OUT.
Rev. 7/16/03
SP6122 Low Voltage, Micro 8, PFET, Buck Controller
14
© Copyright 2003 Sipex Corporation