Datasheet

UCC24610
www.ti.com SLUSA87B –AUGUST 2010–REVISED SEPTEMBER 2010
MODES OF OPERATION
UVLO Mode
When the VCC voltage to the device has not yet reached the V
CC(on)
threshold, or has fallen below the UVLO
threshold V
CC(off)
, the device operates in the low-power UVLO Mode. In this mode, most internal functions are
disabled and ICC current is typically much less than 100 µA. While in this mode, the EN current source is shut
off, an internal 10-kΩ resistance is applied from the EN/TOFF pin to GND, the voltage on EN/TOFF is irrelevant,
and the GATE output is driven low continuously for all VCC > 1.2 V. The device passes out of UVLO Mode when
VCC increases above the V
CC(on)
threshold. UVLO Mode is very similar to Sleep Mode, except VCC current is at
I
CC(start)
level.
Sleep Mode
Sleep Mode is a low-power operating mode similar to UVLO Mode, except that this mode is entered under
external control by forcing V
EN
below the V
EN(off)
threshold. Sleep Mode may be used to reduce device operating
losses to less than 1 mW. VCC current reduces to I
CC(stby)
level. External control overrides any internal timing
conditions, and immediately forces the GATE output low and enters Sleep Mode. Many internal circuits are
turned off to reduce power consumption. When V
EN
is restored to above the V
EN(on)
threshold, the device exits
Sleep Mode synchronously into Light-Load Mode after a delay of approximately 25 µs to allow re-powered
internal circuits to settle.
Run Mode
Run Mode is the normal operating mode of the controller when not in UVLO Mode, Sleep Mode, or Light-Load
Mode. In this mode, VCC current is higher because all internal control and timing functions are operating and the
GATE output is driving the controlled MOSFET for synchronous rectification. VCC current is the sum of I
CC(on)
plus the average current necessary to drive the load on the GATE output. GATE output duty-cycle is dependent
upon system line and load conditions, programmed TON and TOFF times, and SYNC-pulse timing (if applicable).
Light-Load Mode
Light-Load Mode is a low-power operating mode similar to Sleep Mode, except that this mode is entered
automatically based on internal timing conditions. Light-Load Mode automatically reduces switching losses under
light-load conditions by suppressing GATE output pulses whenever the detected synchronous conduction time is
less than the programmed minimum on-time (TON). VCC current reduces to I
CC(on)
level. While in Light-Load
Mode, the MOSFET body-diode conduction time is still continuously monitored. When this time is detected to
once again exceed TON, the device resumes Run Mode on the next switching cycle.
Fault Mode and Other Protections
Fault Mode is a self-protection operating mode of the controller when certain types of single-fault conditions are
detected on certain pins. In this mode, the device enters a shut-down state (not Sleep Mode) and drives the
GATE output Low. Specifically, Fault Mode is entered if R
TON
> 301 kΩ or if R
TON
< 8.7 kΩ. Fault Mode prevents
the conditions of excessive or indefinite on-time (such as from an open-pin) and of excessive TON current (such
as from a shorted-pin).
Similar protection is provided for the EN/TOFF pin. While not specifically detected as faults, if this pin becomes
open-circuited TOFF defaults to a minimum value of ~0.65 µs, and if shorted-to-GND the device enters Sleep
Mode. Additionally, if the SYNC input is continuously held below its trigger threshold voltage, the GATE output is
held low for the entire duration that SYNC remains in that condition.
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