1

OUT

2

R

V

2.5 V

1

R

§

·

˜ ¨

¸

©

¹

OUT

SW(CCM)

10

ON

V

F

1.42 10

R

˜

˜

2

20

OUT

O

SW(DCM)

2

LOAD

ON

V

L

F

10

R

R

˜

˜

˜

8

LM5007

SNVS252H – SEPTEMBER 2003 – REVISED NOVEMBER 2018

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Copyright © 2003–2018, Texas Instruments Incorporated

7.3 Feature Description

7.3.1 Hysteretic Control Circuit Overview

The LM5007 is a buck DC/DC converter that uses a constant on-time (COT) control scheme. The on-time is

programmed by an external resistor and varies inversely with line input voltage (VIN). The core regulation

elements of the LM5007 are the feedback comparator and the programmed on-time one-shot. The regulator

output voltage is sensed at the feedback pin (FB) and compared to an internal reference voltage (2.5 V). If the

FB voltage is below the reference voltage, the buck switch is turned on for a fixed time interval determined by the

input voltage and a programming resistor (RON). Following the on period, the switch remains off for at least the

minimum off-time interval of 300 ns. If the FB voltage is still below the reference after the 300-ns off-time, the

switch turns on again for another on-time interval. This switching behavior continues until the FB voltage reaches

the reference voltage level.

The LM5007 operates in discontinuous conduction mode (DCM) at light load currents and continuous conduction

mode (CCM) at heavier load currents. In DCM, current through the output inductor starts at zero and ramps up to

a peak value during the buck switch on-time and then back to zero during the off-time. The inductor current

remains at zero until the next on-time interval begins when FB falls below the internal reference voltage. The

operating frequency in DCM is relatively low and varies with load. Thus, the conversion efficiency is maintained

at light loads, since the switching losses decrease with the reduction in load current and switching frequency.

Calculate the approximate switching frequency in DCM with Equation 1.

(1)

In CCM, current flows continuously through the inductor and never ramps down to zero. The switching frequency

in CCM is greater than that in DCM and remains relatively constant with load and line variations. Calculate the

approximate switching frequency in CCM with Equation 2.

(2)

The output voltage (VOUT) can be programmed by two external resistors as shown in Figure 4. Calculate the

output voltage setpoint using Equation 3.

(3)

The feedback comparator in hysteretic regulators depend upon the output ripple voltage to switch the power

MOSFET on and off at regular intervals. In order for the internal comparator to respond quickly to changes in

output voltage, proportional to inductor current, a minimum amount of capacitor Equivalent Series Resistance

(ESR) is required. A ripple voltage of 25 mV to 50 mV is recommended at the feedback pin (FB) for stable

operation. In cases where the intrinsic capacitor ESR is too small, additional series resistance may be added.

For applications where lower output voltage ripple is required, the load can be connected directly to the low ESR

output capacitor as shown in Figure 4. The series resistor (R) will degrade the load regulation. Another technique

for enhancing the ripple voltage at FB is to place a capacitor in parallel with the upper feedback resistor, R1. The

addition of this feedforward capacitor reduces the attenuation of the ripple voltage from the feedback divider.

7.3.2 High-Voltage Bias Supply Regulator

The LM5007 contains an internal high-voltage bias supply regulator. The input pin (VIN) can be connected

directly to line voltages from 9 V to 75 V. To avoid supply voltage transients due to long lead inductances on the

input pin (VIN), it is always recommended to connect a low-ESR ceramic capacitor (≈ 0.1 µF) between VIN and

RTN, located close to the respective pins of the LM5007. The bias regulator is internally current limited to 10 mA.

Upon power up, the regulator is enabled and sources current into an external capacitor connected to the VCC pin.

When the VCC voltage reaches the regulation point of 7 V, the controller output is enabled.

An external auxiliary supply voltage can be applied to the VCC pin. If this auxiliary voltage is greater than 7 V, the

internal regulator will essentially shutoff, thus reducing internal power dissipation.