Datasheet

T
OFF
=
V
FB
(6.35 x 10
-6
x R
CL
)
0.285 +
10
-5
STOP
RUN
R
ON
LM5009
RON/SD
VIN
Input
Voltage
LM5009
www.ti.com SNVS402G –FEBRUARY 2006–REVISED FEBRUARY 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 LM5009 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. The on-time equation is:
T
ON
= 1.25 x 10
-10
x R
ON
/ V
IN
(4)
R
ON
should be selected for a minimum on-time (at maximum V
IN
) greater than 250 ns, for proper current limit
operation. This requirement limits the maximum frequency for each application, depending on V
IN
and V
OUT
.
The LM5009 can be remotely disabled by taking the RON/SD pin to ground. See Figure 11. The voltage at the
RON/SD pin is between 1.7V and 5V, depending on Vin and the value of the R
ON
resistor.
Figure 11. Shutdown Implementation
Current Limit
The LM5009 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.31A 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. 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 the following equation:
(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 LM5009 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
0.022‑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 -1V, and the bootstrap capacitor charges from Vcc through
the internal diode. The minimum OFF timer ensures a minimum time each cycle to recharge the bootstrap
capacitor.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 9
Product Folder Links: LM5009