Datasheet

V
IN
V
IN
R
ON
R
ON
7V SERIES
REGULATOR
ON TIMER
V
IN
START
COMPLETE
R
ON
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
-10
x R
ON
/ V
IN
(4)
The R
ON
pin of the LM5007 also provides a shutdown function which disables the regulator and significantly
decreases quiescent power dissipation. By pulling the R
ON
pin to below 0.7V logic threshold activates the low
power shutdown mode. The V
IN
quiescent current in the shutdown mode is approximately 100µA internal to the
LM5007 plus the current in the R
ON
resistor.
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-
resetable current limit off timer is initiated. The duration of the off time is a function of the external resistor (R
CI
)
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
foldback and also reduces the initial start-up time. The current limit off time for a given FB pin voltage and R
CI
resistor can be calculated by the following equation:
(5)
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