LM5161 datasheet(13/37 Pages) TI1

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OFF CL

20 V

4.35

OUT

1.008 x 10

x F

1.008 x 10

x R

LM5161-Q1

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SNVSAF9 – AUGUST 2016

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Feature Description (continued)

7.3.3 Regulation Comparator

The feedback voltage at the FB pin is compared to the SS pin voltage VSS. In normal operation when the output

voltage is in regulation, an ON-time period is initiated when the voltage at FB pin falls below VSS. The high-side

buck switch stays on for the ON-time one-shot period causing the FB voltage to rise. After the on-time period

expires, the high-side switch will remain off until the FB voltage falls below VSS. During start-up, the FB voltage is

below VSS at the end of each on-time period and the high-side switch turns on again after the minimum forced

off-time of 170 ns (typical). When the output is shorted to ground (FB = 0 V), the high side peak current limit is

triggered, the high-side FET is turned off, and remains off for a period determined by the current limit OFF-time

one-shot. See the Current Limit section for additional information.

7.3.4 Soft-Start

The soft-start feature of the LM5161-Q1 allows the converter to gradually reach a steady-state operating point,

thereby reducing start-up stresses and current surges. When the EN/UVLO pin is above the EN/UVLO standby

threshold VUVLO(TH) = 1.24 V (typ) and VCC exceeds the VCC undervoltage VCC(UV) = 3.98 V (typ) threshold, an

internal 10-µA current source charges the external capacitor at the SS pin (CSS) from 0 V to 2 V. The voltage at

the SS pin is connected to the noninverting input of the internal FB comparator. The soft-start interval ends when

the SS capacitor is charged to the 2 V reference level. The ramping voltage at the SS pin produces a controlled,

monotonic output voltage start-up. A minimum 1-nF soft-start capacitor must be used in all applications.

7.3.5 Error Transconductance (GM) Amplifier

The LM5161-Q1 provides a trans-conductance (GM) error amplifier that minimizes the difference between the

reference voltage (VREF) and the average feedback (FB) voltage. This amplifier reduces the load and line

regulation errors that are common in constant-on-time regulators. The soft-start capacitor (CSS) provides

compensation for this error correction loop. The soft-start capacitor should be greater than 1 nF to ensure

stability.

7.3.6 On-Time Generator

The ON-time of the LM5161-Q1 high-side FET is determined by the RON resistor and is inversely proportional to

the input voltage (VIN). The inverse relationship with VIN results in a nearly constant frequency as VIN is varied.

The ON-time can be calculated from Equation 3 with RON expressed in ohms.

(3)

To set a specific continuous conduction mode switching frequency (FSW expressed in Hz), the RON resistor is

determined from Equation 4:

(4)

RON must be selected for a minimum on-time (at maximum VIN) greater than 150 ns for proper operation. This

minimum ON-time requirement limits the maximum switching frequency of applications with relatively high VIN

and low VOUT.

7.3.7 Current Limit

The LM5161-Q1 provides an intelligent current limit OFF-timer that adjusts the OFF-time to reduce foldback of

the current limit. If the peak value of the current in the buck switch exceeds 1.6 A (typical) the present ON-time

period is immediately terminated, and a non-resettable OFF-timer is initiated. The length of the OFF-time is

controlled by the FB voltage and the input voltage VIN. As an example, when VFB = 0.1-V and VIN = 72-V, the

OFF-time is set to 13 μs (typical). This condition would occur if the output is shorted or during the initial phase of

start-up. In cases of output overload where the FB voltage is greater than zero volts (a soft short), the current

limit OFF-time is reduced. Reducing the OFF-time during less severe overloads reduces the current limit

foldback, overload recovery time, and start-up time. The current limit off-time, TOFF(CL) is calculated from

Equation 5:

(5)

LM5161 Datasheet(PDF) 13 Page - Texas Instruments

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