LM5642XMT National Semiconductor, LM5642XMT Datasheet - Page 20

• Manufacturer Part Number: LM5642XMT
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,28PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5642XMT

• Rohs Compliant: NO

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where Tj_max is the maximum allowed junction temperature
in the FET, Ta_max is the maximum ambient temperature,
R
and TC is the temperature coefficient of the on-resistance
which is typically in the range of 4000ppm/°C.
If the calculated R
available, multiple FETs can be used in parallel. This effec-
tively reduces the I
ducing R
calculated R
FET. In the case of three FETs, multiply by 9.
If the selected FET has an Rds value higher than 35.3Ω, then
two FETs with an R
be used in parallel. In this case, the temperature rise on each
FET will not go to Tj_max because each FET is now dissi-
pating only half of the total power.
TOP FET SELECTION
The top FET has two types of losses: switching loss and con-
duction loss. The switching losses mainly consist of crossover
loss and losses related to the low-side FET body diode re-
verse recovery. Since it is rather difficult to estimate the
switching loss, a general starting point is to allot 60% of the
top FET thermal capacity to switching losses. The best way
to precisely determine switching losses is through bench test-
ing. The equation for calculating the on resistance of the top
FET is thus:
Example: Tj_max = 100°C, Ta_max = 60°C, Rqja = 60°C/W,
Vin_min = 5.5V, Vnom = 5V, and Iload_max = 3.6A.
When using FETs in parallel, the same guidelines apply to the
top FET as apply to the bottom FET.
θja
is the junction-to-ambient thermal resistance of the FET,
DS-ON
DS-ON (MAX)
. When using two FETs in parallel, multiply the
DS-ON (MAX)
max
DS-ON
by 4 to obtain the R
term in the above equation, thus re-
less than 141mΩ (4 x 35.3mΩ) can
is smaller than the lowest value
DS-ON (MAX)
for each
(25)
(26)
(27)
(28)
20
Loop Compensation
The general purpose of loop compensation is to meet static
and dynamic performance requirements while maintaining
stability. Loop gain is what is usually checked to determine
small-signal performance. Loop gain is equal to the product
of control-output transfer function and the feedback transfer
function (the compensation network transfer function). Gen-
erally speaking it is desirable to have a loop gain slope that is
roughly -20dB /decade from a very low frequency to well be-
yond the crossover frequency. The crossover frequency
should not exceed one-fifth of the switching frequency. The
higher the bandwidth, the faster the load transient response
speed will be. However, if the duty cycle saturates during a
load transient, further increasing the small signal bandwidth
will not help. Since the control-output transfer function usually
has very limited low frequency gain, it is a good idea to place
a pole in the compensation at zero frequency, so that the low
frequency gain will be relatively large. A large DC gain means
high DC regulation accuracy (i.e. DC voltage changes little
with load or line variations). The rest of the compensation
scheme depends highly on the shape of the control-output
plot.
As shown in Figure 11, the control-output transfer function
consists of one pole (fp), one zero (fz), and a double pole at
fn (half the switching frequency). The following can be done
to create a -20dB /decade roll-off of the loop gain: Place the
first pole at 0Hz, the first zero at fp, the second pole at fz, and
the second zero at fn. The resulting feedback transfer function
is shown in Figure 12.
FIGURE 11. Control-Output Transfer Function
FIGURE 12. Feedback Transfer Function
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