Datasheet

T
OFF
=
V
FB
(6.35 x 10
-6
x R
CL
)
0.285 +
10
-5
STOP
RUN
R
ON
LM5008
R
ON
/SD
V
IN
Input
Voltage
LM5008
www.ti.com SNVS280G –APRIL 2004–REVISED MARCH 2013
Over-Voltage Comparator
The feedback voltage at FB is compared to an internal 2.875V reference. If the voltage at FB rises above 2.875V
the on-time pulse is immediately terminated. This condition can occur if the input voltage, or the output load,
change suddenly. The buck switch will not turn on again until the voltage at FB falls below 2.5V.
On-Time Generator and Shutdown
The on-time for the LM5008 is determined by the R
ON
resistor, and is inversely proportional to the input voltage
(Vin), resulting in a nearly constant frequency as Vin is varied over its range. Equation 4 shows the on-time
equation for the LM5008.
T
ON
= 1.25 x 10
-10
x R
ON
/ V
IN
(4)
See Figure 4. R
ON
should be selected for a minimum on-time (at maximum V
IN
) greater than 400 ns, for proper
current limit operation. This requirement limits the maximum frequency for each application, depending on V
IN
and V
OUT
. See Figure 5.
The LM5008 can be remotely disabled by taking the R
ON
/SD pin to ground. See Figure 12. The voltage at the
R
ON
/SD pin is between 1.5 and 3.0 volts, depending on Vin and the value of the R
ON
resistor.
Figure 12. Shutdown Implementation
Current Limit
The LM5008 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.5A the
present cycle is immediately terminated, and a non-resetable OFF timer is initiated. The length of off-time is
controlled by an external resistor (R
CL
) and the FB voltage (see Figure 6). When FB = 0V, a maximum off-time is
required, and the time is preset to 35µs. 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 95V.
In cases of overload where the FB voltage is above zero volts (not a short circuit) the current limit off-time will be
less than 35µs. Reducing the off-time during less severe overloads reduces the amount of foldback, recovery
time, and the start-up time. The off-time is calculated from Equation 5.
(5)
The current limit sensing circuit is blanked for the first 50-70ns 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 LM5008 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
0.01µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver
during the on-time.
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