7.3.1 Timing Opinion

Three versions of the LM1771 are available each with a predetermined T_ON set internally by EEPROM. This T_ON setting determines the switching frequency for the application. Derivation and calculation of the dependence of the switching frequency on V_IN and T_ON is shown in Equation 1 through Equation 6.

In a PWM buck switcher, Equation 1, Equation 2, and Equation 3 can be manipulated to obtain the switching frequency. Equation 1 shows the standard duty-cycle equation given by the volts-seconds balance on the inductor with Equation 2 and Equation 3 defining standard relationships:

Equation 1.

Equation 3.

Using these equations and solving for duty-cycle for Equation 4:

Frequency can now be expressed in Equation 5:

Equation 5.

Or simply written as Equation 6:

Equation 6.

where

• α = V_IN x T_ON

To maintain a set frequency in an application, α is always held constant by varying T_ON inversely with V_IN. The three versions of the LM1771 are identified by the ON-times at a V_IN of 3.3 V for consistency. For clarification, see Table 1.

The variation of T_ON versus V_IN can also be expressed graphically. These graphs can be found in the Typical Characteristics of the data sheet.

With α being a constant regardless of the version of the LM1771 used, Equation 6 shows that the only dependent variable remaining is V_OUT. Because V_OUT is a constant in any application, the frequency also remains constant. The switching frequency at which the application runs depends upon the V_OUT desired and the LM1771 version chosen. For any V_OUT, three frequency options (LM1771 versions) can be selected. This can be seen Table 2. The recommended frequency range of operation is 100 kHz to 1000 kHz.

7.3.2 Short-Circuit Protection

The LM1771 has an internal short-circuit comparator that constantly monitors the feedback node (except during soft-start). If the feedback voltage drops below 0.55 V (equivalent to the output voltage dropping below 68% of nominal), the comparator trips causing the part to latch off. The LM1771 does not resume switching until the input voltage is taken below the UVLO threshold and then brought back into its normal operating range, or the part is disabled then re-enabled through the enable pin. The purpose of this function is to prevent a severe short circuit from causing damage to the application. Due to the fast transient response of the LM1771 a severe short on the output causing the feedback to drop would only occur if the load applied had an effective resistance that approaches the PMOS R_DS(ON).

7.3.3 Precision Enable

The LM1771 features a precision enable circuit. If the voltage on the EN pin is 1.2 V or greater, the part is enabled and switching occurs. If the enable voltage falls below 1.2 V, the part is placed into a shutdown state and the drivers is tri-stated. This allows the LM1771 to be easily sequenced using a resistive divider from the output of another regulator, or the working input voltage range of the LM1771 to be set using a resistive divider on V_IN. There is no internal pullup connected to the EN pin, so an external signal is required to initiate switching.

7.3.4 Soft-Start

To limit in-rush current and allow for a controlled start-up the LM1771 incorporates an internal soft-start scheme. Every time the enable voltage rises above 1.2 V while V_IN is greater than the UVLO threshold, the LM1771 goes through an adaptive soft-start that limits the ON-time and expands the minimum OFF-time. In addition the part only activates the PMOS allowing a discontinuous mode of operation enabling a prebiased start-up. The time spent in soft-start depends on the load applied to the output, but is usually close to a set time that is dependent on the timing option. The approximate soft-start time can be seen in Table 3 for each timing option.

Therefore, if the input supply is extremely slow rising such that at the end of soft-start the input voltage is still near the UVLO threshold, a timing option must be chosen to ensure that maximum duty-cycle permits the output to meet the minimum condition. TI recommends using the 2000-ns option (LM1771U) in conditions where the output voltage is 2.5 V or greater to avoid false latch-offs when there is concern regarding the input supply slew rate.

In some situations, the internal soft-start routine can create a slight overshoot on the output voltage. If this must be avoided, the use of a feedforward capacitor as detailed in the Feedforward Capacitor section.

7.3.5 Jitter

The LM1771 uses an adaptive ON-time control scheme that relies on the output voltage ripple to provide a consistent switching frequency. Under certain conditions, excessive noise can couple onto the feedback pin causing the switch node to appear to have a slight amount of jitter. This is not indicative of an unstable design. The output voltage still regulates to the exact same value. Careful component selection and layout must minimize any external influence.

In addition to any external noise that can add to the jitter seen on the switch node, the LM1771 always has a slight amount of switch jitter. This is because the LM1771 makes a small alteration in the reference voltage every 128 cycles to improve its accuracy and long-term performance. This has the effect of causing a change in the switching frequency at that instant. When viewed on an oscilloscope this can be seen as a jitter in the switch node. The change in feedback voltage or output voltage, however, is almost indistinguishable.