Datasheet

STOP
RUN
R
T
SM72485
R
T
/SD
VIN
Input
Voltage
SM72485
www.ti.com SNVS697D –JANUARY 2011–REVISED APRIL 2013
On-Time Generator and Shutdown
The on-time for the SM72485 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
SM72485 is:
T
ON
= 1.385 x 10
-10
x R
T
/ V
IN
(4)
R
T
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
.
The SM72485 can be remotely disabled by taking the R
T
/SD pin to ground. See Figure 11. The voltage at the
R
T
/SD pin is between 1.5 and 3.0 volts, depending on Vin and the value of the R
T
resistor.
Figure 11. Shutdown Implementation
Current Limit
The SM72485 contains an intelligent current limit OFF timer. If the current in the Buck switch exceeds 0.3A 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 the following equation:
T
OFF
= 10
-5
/ (0.285 + (V
FB
/ 6.35 x 10
-6
x R
CL
)) (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 SM72485 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 300ns, ensures a minimum time each cycle to recharge the
bootstrap capacitor.
The internal pre-charge switch at the SW pin is turned on for ≊150 ns during the minimum off-time period,
ensuring sufficient voltage exists across the bootstrap capacitor for the on-time. This feature helps prevent
operating problems which can occur during very light load conditions, involving a long off-time, during which the
voltage across the bootstrap capacitor could otherwise reduce below the Gate Drive UVLO threshold. The pre-
charge switch also helps prevent startup problems which can occur if the output voltage is pre-charged prior to
turn-on. After current limit detection, the pre-charge switch is turned on for the entire duration of the forced off-
time.
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