Datasheet

LM2717-ADJ
www.ti.com SNVS407C –DECEMBER 2005–REVISED MARCH 2013
BOOTSTRAP CAPACITOR
A 4.7nF ceramic capacitor or larger is recommended for the bootstrap capacitor. For applications where the input
voltage is less than twice the output voltage a larger capacitor is recommended, generally 0.1µF to 1µF to
ensure plenty of gate drive for the internal switches and a consistently low R
DSON
.
SOFT-START CAPACITOR (BOTH REGULATORS)
The LM2717-ADJ contains circuitry that can be used to limit the inrush current on start-up of the DC/DC
switching regulators. This inrush current limiting circuitry serves as a soft-start. The external SS pins are used to
tailor the soft-start for a specific application. A current (I
SS
) charges the external soft-start capacitor, C
SS
. The
soft-start time can be estimated as:
T
SS
= C
SS
*0.6V/I
SS
(7)
When programming the soft-start time use the equation given in the Soft-Start Capacitor section above. The soft-
start function is used simply to limit inrush current to the device that could stress the input voltage supply. The
soft-start time described 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 voltage and the
shutdown pin to protect the device.
SCHOTTKY DIODE
The breakdown voltage rating of D
1
and D
2
is preferred to be 25% higher than the maximum input voltage. The
current rating 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, approximately (1-D)*I
OUT
however the peak current rating should be higher 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 function). 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 switching 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 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 17, 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 transfer function is shown in Figure 18.
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