Datasheet

:R
FB1
= R
FB2
x
V
OUT
- 1.267
1.267
V
OUT
V
IN
, D' = (1-D)
D =
LM2717
SNVS253D –MAY 2005–REVISED MARCH 2013 www.ti.com
BUCK OPERATION
PROTECTION (BOTH REGULATORS)
The LM2717 has dedicated protection circuitry running during normal operation to protect the IC. The Thermal
Shutdown circuitry turns off the power devices when the die temperature reaches excessive levels. The UVP
comparator protects the power devices during supply power startup and shutdown to prevent operation at
voltages less than the minimum input voltage. The OVP comparator is used to prevent the output voltage from
rising at no loads allowing full PWM operation over all load conditions. The LM2717 also features a shutdown
mode for each converter decreasing the supply current to approximately 10µA (both in shutdown mode).
CONTINUOUS CONDUCTION MODE
The LM2717 contains current-mode, PWM buck regulators. A buck regulator steps the input voltage down to a
lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady
state), the buck regulator operates in two cycles. The power switch is connected between V
IN
and SW1 and
SW2.
In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the
inductor and the load current is supplied by C
OUT
and the rising current through the inductor.
During the second cycle the transistor is open and the diode is forward biased due to the fact that the inductor
current cannot instantaneously change direction. The energy stored in the inductor is transferred to the load and
output capacitor.
The ratio of these two cycles determines the output voltage. The output voltage is defined approximately as:
(1)
where D is the duty cycle of the switch, D and D′ will be required for design calculations.
DESIGN PROCEDURE
This section presents guidelines for selecting external components.
SETTING THE OUTPUT VOLTAGE (ADJUSTABLE REGULATOR)
The output voltage is set using the feedback pin and a resistor divider connected to the output as shown in
Figure 12. The feedback pin voltage is 1.26V, so the ratio of the feedback resistors sets the output voltage
according to the following equation:
(2)
INPUT CAPACITOR
A low ESR aluminum, tantalum, or ceramic capacitor is needed betwen the input pin and power ground. This
capacitor prevents large voltage transients from appearing at the input. The capacitor is selected based on the
RMS current and voltage requirements. The RMS current is given by:
(3)
The RMS current reaches its maximum (I
OUT
/2) when V
IN
equals 2V
OUT
. This value should be calculated for both
regulators and added to give a total RMS current rating. For an aluminum or ceramic capacitor, the voltage rating
should be at least 25% higher than the maximum input voltage. If a tantalum capacitor is used, the voltage rating
required is about twice the maximum input voltage. The tantalum capacitor should be surge current tested by the
manufacturer to prevent being shorted by the inrush current. The minimum capacitor value should be 47µF for
lower output load current applications and less dynamic (quickly changing) load conditions. For higher output
current applications or dynamic load conditions a 68µF to 100µF low ESR capacitor is recommended. It is also
recommended to put a small ceramic capacitor (0.1µF to 4.7µF) between the input pins and ground to reduce
high frequency spikes.
8 Submit Documentation Feedback Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: LM2717