F =

1.42 x 10

ON

F =

1 x 10

2

LM5007

SNVS252G – SEPTEMBER 2003 – REVISED NOVEMBER 2015

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Feature Description (continued)

The LM5007 operates in discontinuous conduction mode at light load currents or continuous conduction mode at

heavier load currents. In discontinuous conduction mode, 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 period starts when FB falls below the internal reference. In

discontinuous mode the operating frequency can be relatively low and will vary with load. Therefore at light loads

the conversion efficiency is maintained, since the switching losses decrease with the reduction in load current

and switching frequency. The approximate discontinuous mode operating frequency can be calculated as follows:

(1)

In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero.

In this mode the operating frequency is greater than the discontinuous mode frequency and remains relatively

constant with load and line variations. The approximate continuous mode operating frequency can be calculated

as follows:

(2)

point can be calculated as follows:

(3)

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

transistor 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 the FB pin is to place a capacitor in parallel with the feedback

divider resistor R1. The addition of the capacitor reduces the attenuation of the ripple voltage from the feedback

divider

7.3.2 High Voltage Bias Regulator

Pin 8), it is always recommended to connect low ESR ceramic chip capacitor (

RTN pin (pin 4), located close to LM5007. The regulator is internally current limited to 10 mA. Upon power up,

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

8

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