Datasheet

T
OFF
=
(V
IN
+ 1.83V) x 0.28
(V
FB
x 1.05) + 0.58
STOP
RUN
R
T
LM5006
R
T
/SD
VIN
Input
Voltage
T
ON
=
1.25 x 10
-10
x (R
T
+ 500:) + 30 ns
(V
IN
- 0.5V)
LM5006
www.ti.com SNVS646B –FEBRUARY 2011–REVISED MARCH 2013
On-Time Generator and Shutdown
The on-time for the LM5006 is determined by the R
T
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
LM5006 is:
(5)
R
T
should be selected for a minimum on-time (at maximum V
IN
) greater than 200 ns, for proper current limit
operation. This requirement limits the maximum frequency for each application, depending on V
IN
and V
OUT
.
The LM5006 can be remotely disabled by taking the RT/SD pin to ground. See Figure 34. The voltage at the
RT/SD pin is between 1.5 and 5.0 volts, depending on Vin and the value of the R
T
resistor.
Figure 34. Shutdown Implementation
Current Limit
The LM5006 contains an intelligent current limit OFF timer. If the current in the Buck switch reaches the current
limit threshold, the present cycle is immediately terminated, and a non-resetable OFF timer is triggered. The
length of off-time is controlled by the FB voltage and V
IN
(see Figure 8). When FB = 0V, a maximum off-time is
required. 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 75V. In cases of overload where the
FB voltage is above zero volts (not a short circuit) the required current limit off-time is less. Reducing the off-time
during less severe overloads reduces the amount of foldback, recovery time, and the start-up time. The off-time
in microseconds is calculated from the following equation:
(6)
See Figure 8.
The current limit sensing circuit is blanked for the first 50-70 ns 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 LM5006 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 0.01
µF ceramic capacitor (C4) connected between the BST pin and SW pin provides the voltage to the driver during
the on-time.
During each off-time, the SW pin is at approximately 0V, and the bootstrap capacitor charges from Vcc through
the internal diode. The minimum OFF timer, set to 260 ns, ensures a minimum time each cycle to recharge the
bootstrap capacitor.
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