UVLO

7 V

SW Pin

Inductor

Current

Vin

t 2

I

LIM

V

CC

V

IN

t1

t 3

I

O

0 V

V

OUT

10

LM5008A

SNVS583H – MARCH 2009 – REVISED OCTOBER 2018

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Feature Description (continued)

When VIN exceeds the bypass threshold, the time required for Q2 to shut off is approximately 2 µs to 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 placed 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 7.5 V and 14 V 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

Figure 6. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than VIN.

The turnon sequence is shown in Figure 8. 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 undervoltage

lockout (UVLO, typically 5.3 V), the buck switch is enabled. The inductor current increases to the current limit

threshold (ILIM), and during t2 the 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).

Figure 8. Start-Up Sequence

7.3.3 Regulation Comparator

The feedback voltage at FB is compared to an internal 2.5-V reference. In normal operation (the output voltage is

regulated), an on-time period is initiated when the voltage at FB falls below 2.5 V. The buck switch stays on for

the on-time, causing the FB voltage to rise above 2.5 V. After the on-time period, the buck switch stays off until

the FB voltage again falls below 2.5 V. During start-up, the FB voltage is below 2.5 V at the end of each on-time,

resulting in the minimum off-time of 300 ns. Bias current at the FB pin is nominally 100 nA.

7.3.4 Overvoltage Comparator

The feedback voltage at FB is compared to an internal 2.875-V reference. If the voltage at FB rises above 2.875

V, the on-time pulse is immediately terminated. This condition can occur if the input voltage or the output load

change suddenly. The buck switch does not turn on again until the voltage at FB falls below 2.5 V.

7.3.5 On-Time Generator and Shutdown

The on-time for the LM5008A 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

LM5008A is Equation 4.