:

1.267

, 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

SW2.

In the first cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the

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)

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

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