LM2717-ADJ_08 NSC [National Semiconductor], LM2717-ADJ_08 Datasheet - Page 11

• Manufacturer Part Number: LM2717-ADJ_08
• Description: Dual Step-Down DC/DC Converter
• Manufacturer: NSC [National Semiconductor]

could stress the input voltage supply. The soft-start time de-
scribed above is the time it takes for the current limit to ramp
to maximum value. When this function is used the current limit
starts at a low value and increases to nominal at the set soft-
start time. Under maximum load conditions the output voltage
may rise at the same rate as the soft-start, however at light or
no load conditions the output voltage will rise much faster as
the switch will not need to conduct much current to charge the
output capacitor.
SHUTDOWN OPERATION (BOTH REGULATORS)
The shutdown pins of the LM2717-ADJ are designed so that
they may be controlled using 1.8V or higher logic signals. If
the shutdown function is not to be used the pin may be left
open. The maximum voltage to the shutdown pin should not
exceed 7.5V. If the use of a higher voltage is desired due to
system or other constraints it may be used, however a 100k
or larger resistor is recommended between the applied volt-
age and the shutdown pin to protect the device.
SCHOTTKY DIODE
The breakdown voltage rating of D
25% higher than the maximum input voltage. The current rat-
ing for the diode should be equal to the maximum output
current for best reliability in most applications. In cases where
the input voltage is much greater than the output voltage the
average diode current is lower. In this case it is possible to
use a diode with a lower average current rating, approximate-
ly (1-D)*I
than the maximum load current.
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 output-control
transfer function (the compensation network transfer func-
tion). The DC loop gain of the LM2717 is usually around 55dB
to 60dB when loaded. Generally speaking it is a good idea to
have a loop gain slope that is -20dB /decade from a very low
frequency to well beyond the crossover frequency. The
crossover frequency should not exceed one-fifth of the
switching frequency, i.e. 60kHz in the case of 300kHz switch-
ing frequency. The higher the bandwidth is, the faster the load
transient response speed will potentially 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 rela-
tively large. A large DC gain means high DC regulation ac-
curacy (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.
OUT
however the peak current rating should be higher
1
and D
2
is preferred to be
11
As shown in Figure 1, the example 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 output-control trans-
fer function is shown in Figure 2.
The control-output corner frequencies, and thus the desired
compensation corner frequencies, can be determined ap-
proximately by the following equations:
Where C
tance ESR, R
and f is the switching frequency used.
Since fp is determined by the output network, it will shift with
loading (Ro) and duty cycle. First determine the range of fre-
quencies (fpmin/max) of the pole across the expected load
range, then place the first compensation zero within that
range.
Example: V
mA = 50Ω, R
FIGURE 1. Control-Output Transfer Function
FIGURE 2. Output-Control Transfer Function
o
is the output capacitance, R
o
= 5V, R
omin
o
is the load resistance, L is the inductor value,
= 5V/1A = 5Ω, L = 10µH, f = 300kHz:
e
= 20mΩ, C
o
= 100µF, R
e
is the output capaci-
omax
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20167916
= 5V/100-
20167917