LM5009AMMSLASHNOPB datasheet(7/24 Pages) TI

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RFB2

VOUT2

LM5009A

RFB1

F =

VOUT

1.385 x 10

-10 x RT

LM5009A

www.ti.com

SNVS608G – JUNE 2009 – REVISED FEBRUARY 2013

where RL = the load resistance

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)

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

regulation point can be calculated as follows:

VOUT = 2.5 x (RFB1 + RFB2) / RFB1

(3)

The LM5009A 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 25mV to 50mV of ripple voltage at the feedback pin

(FB) is required for the LM5009A. In cases where the capacitor ESR is too small, additional series resistance

may be required (R3 in the 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 9. However, R3 slightly degrades the load regulation.

Figure 9. Low Ripple Output Configuration

Start-Up Regulator (VCC)

The high voltage bias regulator is integrated within the LM5009A. The input pin (VIN) can be connected directly

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

graph of VCC vs VIN, when VIN is between 6V and the bypass threshold (nominally 8.5V), the bypass switch (Q2)

is on, and VCC tracks VIN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 100Ω, with

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

VCC is regulated at 7V. The VCC regulator output current is limited at approximately 9.2 mA. When the LM5009A

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

When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 - 3 µs. The

capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its

absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to

the VCC and RTN pins. In applications with a relatively high input voltage, power dissipation in the bias regulator

is a concern. An auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin to shut off

the VCC regulator, thereby reducing internal power dissipation. The current required into the VCC pin is shown in

the graph “ICC Current vs. Applied VCC Voltage”. Internally a diode connects VCC to VIN requiring that the

auxiliary voltage be less than VIN.

The turn-on sequence is shown in Figure 10. During the initial delay (t1) VCC ramps up at a rate determined by

its current limit and C3 while internal circuitry stabilizes. When VCC reaches the upper threshold of its under-

voltage lock-out (UVLO, typically 5.3V) the buck switch is enabled. The inductor current increases to the current

limit threshold (ILIM) and during t2 VOUT increases as the output capacitor charges up. When VOUT reaches the

intended voltage the average inductor current decreases (t3) to the nominal load current (IO).

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