LM5006MMXSLASHNOPB datasheet(13/27 Pages) TI

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TOFF =

(VIN + 1.83V) x 0.28

(VFB x 1.05) + 0.58

STOP

RUN

LM5006

RT/SD

VIN

Input

Voltage

TON =

1.25 x 10

-10 x (R

T + 500

:) + 30 ns

(VIN - 0.5V)

LM5006

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SNVS646B – FEBRUARY 2011 – REVISED MARCH 2013

On-Time Generator and Shutdown

The on-time for the LM5006 is determined by the RT resistor, and is inversely proportional to the input voltage

(Vin), resulting in a nearly constant frequency as Vin is varied over its range. The on-time equation for the

LM5006 is:

(5)

RT should be selected for a minimum on-time (at maximum VIN) greater than 200 ns, for proper current limit

operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT.

The LM5006 can be remotely disabled by taking the RT/SD pin to ground. See Figure 34. The voltage at the

RT/SD pin is between 1.5 and 5.0 volts, depending on Vin and the value of the RT resistor.

Figure 34. Shutdown Implementation

Current Limit

The LM5006 contains an intelligent current limit OFF timer. If the current in the Buck switch reaches the current

limit threshold, the present cycle is immediately terminated, and a non-resetable OFF timer is triggered. The

length of off-time is controlled by the FB voltage and VIN (see Figure 8). When FB = 0V, a maximum off-time is

required. This condition occurs when the output is shorted, and during the initial part of start-up. This amount of

time ensures safe short circuit operation up to the maximum input voltage of 75V. In cases of overload where the

FB voltage is above zero volts (not a short circuit) the required current limit off-time is less. Reducing the off-time

during less severe overloads reduces the amount of foldback, recovery time, and the start-up time. The off-time

in microseconds is calculated from the following equation:

(6)

See Figure 8.

The current limit sensing circuit is blanked for the first 50-70 ns of each on-time so it is not falsely tripped by the

current surge which occurs at turn-on. The current surge is required by the re-circulating diode (D1) for its turn-

off recovery.

N - Channel Buck Switch and Driver

The LM5006 integrates an N-Channel Buck switch and associated floating high voltage gate driver. The gate

driver circuit works in conjunction with an external bootstrap capacitor and an internal high voltage diode. A 0.01

µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during

the on-time.

During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through

the internal diode. The minimum OFF timer, set to 260 ns, ensures a minimum time each cycle to recharge the

bootstrap capacitor.

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