SP6123 Datasheet, PDF (11/18 Pages) Sipex Corporation – Low Voltage, Synchronous Step-Down PWM Controller Ideal for 2A to 10A, Small Footprint, DC-DC Power Converters

power sources. Certain tantalum capacitors, such
as AVX TPS series, are surge tested. For generic
tantalum capacitors, use 2:1 voltage derating to
protect the input capacitors from surge fallout.
MOSFET Selection
The losses associated with MOSFETs can be
divided into conduction and switching losses.
Conduction losses are related to the on resis-
tance 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 an external
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, making the assumption that the
turn on and turn off transition times are equal,
the transition time can be approximated by:
t_T = ^CISSVIN _,
I_G
where C_ISS is the MOSFET’s input capacitance,
or the sum of the gate-to-source capacitance,
C_GS, and the drain-to-gate capacitance, C_GD.
This parameter can be directly obtained from
the MOSFET’s data sheet.
I_G is the gate drive current provided by the
SP6123 (approximately 1A at V_IN=5V) and V_IN
is the input supply voltage.
Therefore an approximate expression for the
switching losses associated with the high side
MOSFET can be given as:
P_SH(max) = (V_IN(max) + V_F)I_OUT(max)t_TF_S ,
where t_T is the switching transition time and V_F
is the free wheeling diode drop.
Switching losses need to be taken into account
for high switching frequency, since they are
directly proportional to switching frequency.
The conduction losses associated with top and
bottom MOSFETs are determined by
P_CH(max) = R_DS(ON)I_OUT(max)2D
APPLICATIONS INFORMATION
P_CL(max) = R_DS(ON)I_OUT(max)2(1 - D),
where:
P_CH(max) = conduction losses of the high side
MOSFET
P_CL(max) = conduction losses of the low side
MOSFET
R_DS(ON) = drain to source on resistance.
The total power losses of the top MOSFET are
the sum of switching and conduction losses. For
synchronous 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 domi-
nate switching losses. Therefore, lowering the
R_DS(ON) of the MOSFETs always improves effi-
ciency even though it gives rise to higher switch-
ing losses due to increased C_ISS .
Total gate charge is the charge required to turn
the MOSFETs on and off under the specified
operating conditions (V_GS and V_DS). The gate
charge is provided by the SP6123 gate drive
circuitry. (At 500kHz switching frequency, the
gate charge is the dominant source of power
dissipation in the SP6123). At low output levels,
this power dissipation is noticeable as a reduc-
tion in efficiency. The average current required
to drive the high side and low side MOSFETs is:
I_G(av) = Q_GHF_S + Q_GLF_S, where
Q_GH and Q_GL are the total charge for the high
side and the low side MOSFETs respectively.
Considering that the gate charge current comes
from the input supply voltage V_IN, the power
dissipated in the SP6123 due to the gate drive is:
P_{GATE DRIVE} = V_INI_G(av)
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 volt-
age. 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
Date: 5/25/04
SP6123 Low Voltage, Synchronous Step Down PWM Controller
11
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