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FB

SW

L1

C2

RFB2

VOUT2

R3

LM5008A

RFB1

OUT

10

T

V

F

1.385 10

R

u

u

2

20

OUT

2

L

T

V

L 1.04 10

F

R

(R )

u u

u

u

9

LM5008A

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SNVS583H – MARCH 2009 – REVISED OCTOBER 2018

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Feature Description (continued)

The LM5008A operates in discontinuous conduction mode at light load currents, and continuous conduction

mode at heavy load current. In discontinuous conduction mode, current through the output inductor starts at zero

and ramps up to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-

time period starts when the voltage at FB falls below the internal reference; until then, the inductor current

remains zero. In this mode, the operating frequency is lower than in continuous conduction mode and varies with

load current. Therefore, at light loads, the conversion efficiency is maintained because the switching losses

reduce with the reduction in load and frequency. The discontinuous operating frequency can be calculated with

Equation 1.

where

•

RL = the load resistance

(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

with Equation 2.

(2)

The output voltage (VOUT) is programmed by two external resistors as shown in the Functional Block Diagram.

The regulation point can be calculated with Equation 3.

VOUT = 2.5 × (RFB1 + RFB2) / RFB1

(3)

The LM5008A regulates the output voltage based on ripple voltage at the feedback input, requiring a minimum

amount of ESR for the output capacitor C2. A minimum of 25 mV to 50 mV of ripple voltage at the feedback pin

(FB) is required for the LM5008A. In cases where the capacitor ESR is too small, additional series resistance

may be required (R3 in the Functional Block Diagram).

For applications where lower output voltage ripple is required, the output can be taken directly from a low-ESR

output capacitor as shown in Figure 7. However, R3 slightly degrades the load regulation.

Figure 7. Low-Ripple Output Configuration

7.3.2 Start-Up Regulator (VCC)

The high voltage bias regulator is integrated within the LM5008A. The input pin (VIN) can be connected directly

to line voltages between 6 V and 95 V, with transient capability to 100 V. Referring to the Functional Block

Diagram and Figure 2, when VIN is between 6 V and the bypass threshold (nominally 8.5 V), the bypass switch

(Q2) is on, and VCC tracks VIN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100

Ω, with inherent current limiting at approximately 100 mA. When VIN is above the bypass threshold Q2 is turned

off, and VCC is regulated at 7 V. The VCC regulator output current is limited at approximately 9.2 mA. When the

LM5008A is shut down using the RT/SD pin, the VCC bypass switch is shut off regardless of the voltage at VIN.