Datasheet

EN / TOFF
s
TOFF( s) 11( s) 39 R (M ) 0.65( s)(min)
M
æ ö
m
æ ö
m = m - W + m
ç ÷
ç ÷
W
è ø
è ø
EN / TOFF
0.1 R (M ) 0.282£ W £
( )
EN / TOFF
11( s) 0.65( s)(min) TOFF( s)
R (M )
s
39
M
m + m - m
W =
m
æ ö
ç ÷
W
è ø
0.65 TOFF( s) 7.75£ m £
UCC24610
www.ti.com SLUSA87B –AUGUST 2010–REVISED SEPTEMBER 2010
Enabling and TOFF Programming
The controller must be out of UVLO Mode, or the internal current source on EN/TOFF pin is shut off and the pin
is pulled low with an internal 10-kΩ resistor. Before the device is in the Enabled state, the current source on
EN/TOFF delivers 20 mA. Prudent design practice indicates that a minimum R
EN/TOFF
value of 93 kΩ is necessary
to ensure the pin voltage exceeds the disable threshold. After being Enabled, the Enabled state is latched and
the source current reduces to 10 mA. This current level establishes the voltage which determines the TOFF time,
as programmed below.
Once both the VCC and EN/TOFF conditions are met to enable the device, an internal power-up sequence
ensures that the controller starts the SR-MOSFET synchronously with the system conduction conditions. This
avoids turn-on of the SR-MOSFET into an inappropriate system state. After a ~25-ms delay to allow internal
references to stabilize, SR operation commences in Light-Load Mode and the load condition is monitored at the
first complete switching cycle after the delay to determine the next operating mode.
Because V
DS
of the SR-MOSFET may ring above 1.5 V and back below -150 mV one or more times (due to
circuit parasitic elements), TOFF time should be programmed to block GATE re-arming for the duration of this
ringing. In a system, the duration of this ringing may be unknown until actual prototypes are operational and
observable, so a longer TOFF time may be initially programmed and the final value adjusted after system
evaluation and optimization.
Nominal TOFF off time is programmed by the following formula, where TOFF is in ms and R
EN/TOFF
is in MΩ:
(2)
valid for:
(3)
Conversely,
(4)
valid for:
(5)
For any R
EN/TOFF
> 282 kΩ, TOFF = 0.65 ms.
For any 70 kΩ < R
EN/TOFF
< 80 kΩ, V
EN
toggles rapidly between 1.4 V and 0.8 V and the device remains
disabled. In this situation, average I
CC
is approximately half of the normal Run-Mode current, I
CC(on)
.
For any R
EN/TOFF
< 70 kΩ, VEN is < 1.4 V and the device is disabled, operating in Sleep Mode.
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