Datasheet

UCC24610
www.ti.com SLUSA87B –AUGUST 2010–REVISED SEPTEMBER 2010
TERMINAL FUNCTIONS
TERMINAL
I/O DESCRIPTION
NAME NO.
EN/TOFF (Combined Enable Function & Programmable Off-Time Timer)
When VCC falls below the V
CC(off)
threshold, the UCC24610 is in UVLO Mode, the EN/TOFF
input is internally connected to GND through a 10-kΩ resistance and the internal current
source is turned off. When VCC exceeds the V
CC(on)
threshold, the 10-kΩ resistance is
removed and the current source is turned on. Thereafter, when EN/TOFF exceeds V
EN(on)
,
the UCC24610 is in Run Mode and when EN/TOFF falls below V
EN(off)
, the UCC24610 is in
Sleep Mode. The voltage level on EN/TOFF also programs the minimum off-time (T
OFF
) for
the controlled MOSFET. EN/TOFF is internally driven by a two-level current source, so the
EN/TOFF 2 I
voltage level on EN/TOFF can be set by connecting a resistor from EN/TOFF to GND. The
EN/TOFF current source initially drives twice as much current (I
EN-START
) to achieve the
enable threshold voltage V
EN(on)
, and then drops to the normal Run Mode level (I
EN-ON
) to
program the T
OFF
time. Alternatively, the desired EN/TOFF voltage may be forced using an
external source. The T
OFF
time is programmed to suppress GATE output for a desired
duration to avoid possible false retriggering from resonant ringing or noise after turn-off. The
T
OFF
timer is triggered when VD voltage exceeds 1.5 V after GATE transitions from high to
low.
GATE (Controlled MOSFET Gate Drive)
Connect GATE to the gate of the controlled MOSFET through a small series resistor using
short PC board tracks to achieve optimal switching performance. The GATE output can
GATE 5 O achieve >1-A peak source current when High and >2-A peak sink current when Low into a
large N-channel power MOSFET.
In Sleep Mode and UVLO, GATE impedance to GND is about 1.6 Ω.
GATE impedance to GND crests about 80 Ω, when VCC ≈ 1.1 V.
GND (Combined Analog and Power Ground)
This ground input is the reference potential for the GATE driver, the UVLO comparator, the
GND 6 EN/TOFF comparator, the EN/TOFF timer, and the TON timer. Connect a 0.1-µF or larger
ceramic bypass capacitor from the VCC pin to the GND pin through very short PC-board
tracks.
PowerPad™ (Thermal Pad on QFN package only)
The exposed pad (PowerPad™) on the bottom of the QFN package enhances the thermal
performance of the device, and is intended to be soldered to a heat-dissipating pad on the
PowerPad™ 9
PCB. This pad should be connected to the GND pin, or may be left floating (unconnected to
any network). It is internally connected to GND through an indeterminate impedance and so
may not be used to carry current.
SYNC (Gate Turn-Off Synchronization)
A falling edge on SYNC immediately forces GATE low, turning off the controlled MOSFET
asynchronous to the voltage on the drain and source, and regardless of the state of the TON
timer. When a power converter is operated in Continuous Conduction Mode (CCM), it is
SYNC 1 I
necessary to turn off the controlled MOSFET under command of the switching converter.
Connect SYNC to a control signal on the primary side of the converter using a high-voltage
isolation capacitor or transformer, or other suitable coupling means. A continuous low level
on the SYNC input causes GATE to be driven low for the same duration.
TON (Programmable On-Time Timer)
Program the minimum on time with a resistor from TON to GND. When the controlled
MOSFET gate is turned on, some ringing noise is generated. The minimum on-time timer
blanks the VD-VS comparator, keeping the controlled MOSFET on for at least the
TON 3 I programmed minimum time. This time also determines the light-load shut-down point. If
VD-VS falls below the -5-mV threshold before TON time expires, the controller transitions
into Light-Load Mode on the next switching cycle. When VD-VS falls below the -5-mV
threshold after TON expires, the device resumes Run-Mode operation on the next switching
cycle.
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