Current Limit

The LM5007 contains an intelligent current limit off timer

intended to reduce the foldback characteristic inherent with

fixed off-time over-current protection. If the current in the

Buck switch exceeds 725mA the present cycle on time is

immediately terminated (cycle by cycle current limit). Follow-

ing the termination of the cycle a non-resetable current limit

off timer is initiated. The duration of the off time is a function

of the external resistor (R

the FB pin voltage equals zero, the current limit off time is

internally preset to 17uS. This condition occurs in short

circuit operation when a maximum amount of off time is

required.

In cases of overload (not complete short circuit) the current

limit off time can be reduced as a function of the output

voltage (measured at the FB pin). Reducing the off time with

smaller overloads reduces the amount of foldback and also

reduces the initial start-up time. The current limit off time for

a given FB pin voltage and R

the following equation:

Toff=10

Applications utilizing low resistance inductors and/or a low

voltage drop rectifier may require special evaluation at high

line, short circuit conditions. In this special case the preset

17uS (FB = 0V) off time may be insufficient to balance the

inductor volt*time product. Additional inductor resistance,

output resistance or a larger voltage drop rectifier may be

necessary to balance the inductor cycle volt*time product

and limit the short circuit current.

N - Channel Buck Switch and

Driver

The LM5007 integrates an N-Channel Buck switch and as-

sociated floating high voltage gate driver. This gate driver

circuit works in conjunction with an external bootstrap ca-

pacitor and an internal high voltage diode. The bootstrap

capacitor is charged by V

diode. A 0.01uF ceramic capacitor connected between the

BST pin and SW pin is recommended.

During each cycle when the Buck switch turns off, the SW

pin is approximately 0V. When the SW pin voltage is low, the

bootstrap capacitor will be charged from Vcc through the

internal diode. The minimum off timer, set to 300ns, ensures

that there will be a minimum interval every cycle to recharge

the bootstrap capacitor.

An external re-circulating diode from the SW pin to ground is

necessary to carry the inductor current after the internal

Buck switch turns off. This external diode must be of the

Ultra-fast or Schottky type to reduce turn-on losses and

current over-shoot. The reverse voltage rating of the re-

circulating diode must be greater than the maximum line

input voltage.

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the

integrated circuit in the event the maximum junction tem-

perature is exceeded. When thermal protection is activated,

typically at 165 degrees Celsius, the controller is forced into

a low power reset state, disabling the output driver. This

feature is provided to prevent catastrophic failures from ac-

cidental device overheating.

20078307

FIGURE 3. Shutdown Implementation

www.national.com

8