SP6134 Datasheet, PDF (10/14 Pages) Sipex Corporation – Dual Supply Synchronous Buck Controller

APPLICATIONS INFORMATION: Continued
The capacitor type suitable for the output capac-
itors can also be used for the input capacitors.
However, exercise extra caution when tantalum
capacitors are considered. Tantalum capacitors are
known for catastrophic failure when exposed to
surge current, and input capacitors are prone to
such surge current when power supplies are con-
nected “live” to low impedance power sources.
MOSFET Selection
The total power losses of the top MOSFET are the
sum of switching and conduction losses. For syn-
chronous buck converters of efficiency over 90%,
allow no more than 4% power losses for high or
low side MOSFETs. For input voltages of 3.3V
and 5V, conduction losses often dominate switch-
ing losses. Therefore, lowering the R_DS(ON) of the
MOSFETs always improves efficiency even
though it gives rise to higher switching losses due
to increased C_rss.
The losses associated with MOSFETs can be
divided into conduction and switching losses.
Conduction losses are related to the on resistance
of MOSFETs, and increase with the load current.
Switching losses occur on each on/off transition
when the MOSFETs experience both high current
and voltage. Since the bottom MOSFET switches
current from/to a paralleled diode (either its own
body diode or a Schottky diode), the voltage across
the MOSFET is no more than 1V during switching
transition. As a result, its switching losses are
negligible. The switching losses are difficult to
quantify due to all the variables affecting turn on/
off time. However, the following equation pro-
vides an approximation on the switching losses
associated with the top MOSFET driven by SP6134.
P_{SH (max)} = 12C _rssV _{IN (max)}I _{OUT (max)}F_S
where
C_rss = reverse transfer capacitance of the top
MOSFET
Switching losses need to be taken into account for
high switching frequency, since they are directly
proportional to switching frequency. The conduc-
tion losses associated with top and bottom
MOSFETs are determined by:
P = R I D _{CH (max)}
2
DS (ON ) OUT (max)
Top and bottom MOSFETs experience unequal
conduction losses if their on time is unequal. For
applications running at large or small duty cycle, it
makes sense to use different top and bottom
MOSFETs. Alternatively, parallel multiple
MOSFETs to conduct large duty factor.
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 SP6134 in 3.3V only
applications.
Thermal calculation must be conducted to ensure
the MOSFET can handle the maximum load cur-
rent. The junction temperature of the MOSFET,
determined as follows, must stay below the maxi-
mum rating.
T = T + ^P _{R J (max)}
A (max)
MOSFET (max)
θ JA
P_CL(max) = R _{DS(ON )}I _{OUT (max)2}(1 − D)
where
P_CH(max) = conduction losses of the high side
MOSFET
where
T_A(max) = maximum ambient temperature
PMOSFET(max) = maximum power dissipa-
tion of the MOSFET
R_ΘJA = junction to ambient thermal resistance.
P_CL(max) = conduction losses of the low side
MOSFET
R_DS(ON) = drain to source on resistance.
R_ΘJA of the device depends greatly on the board
layout, as well as device package. Significant
Date: 5/25/04
SP6134 Dual Supply, Synchronous Buck Controller
© Copyright 2004 Sipex Corporation
10