7V SERIES

REGULATOR

ON TIMER

START

COMPLETE

STOP

RUN

LM5007

LM5007

www.ti.com

SNVS252F – SEPTEMBER 2003 – REVISED MARCH 2013

Over-Voltage Comparator

The over-voltage comparator is provided to protect the output from overvoltage conditions due to sudden input

line voltage changes or output loading changes. The over-voltage comparator monitors the FB pin versus an

internal 2.875V reference (OV_REF). If the voltage at FB rises above OV_REF the comparator immediately

terminates the buck switch on time pulse.

ON Time generator and Shutdown

The on time of the LM5007 is set inversely proportional to the input voltage by an external resistor connected

between Ron and Vin. The Ron terminal is a low impedance input biased at approximately 1.5V. Thus the

current through the resistor and into the Ron terminal is approximately proportional to Vin and used internally to

control the on timer. This scheme of input voltage feed-forward hysteretic operation achieves nearly constant

operational frequency over varying line and load conditions. The on time equation for the LM5007 is :

Ton = 1.42 x 10

(4)

Figure 4. Shutdown Implementation

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). Following the termination of the cycle a non-

and the FB pin voltage. When 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

resistor can be calculated by the following equation:

(5)

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