Datasheet

SW
CURRENT LIMIT
COMPARATOR
CS
RAMP
CLK
CSG
+
-
+
-
1.2V
CURRENT SENSE
AMPLIFIER
A=10
HO
C
RAMP
R
RAMP
R
S
I
L
LM5119/LM5119Q
www.ti.com SNVS676F –AUGUST 2010–REVISED FEBRUARY 2013
Figure 5. Current Limit and Ramp Circuit
Hiccup Mode Current Limiting
To further protect the regulator during prolonged current limit conditions, an internal counter counts the PWM
clock cycles during which cycle-by-cycle current limiting occurs. When the counter detects 256 consecutive
cycles of current limiting, the regulator enters a low power dissipation hiccup mode with the HO and LO outputs
disabled. The restart timer pin, RES, and an external capacitor configure the hiccup mode current limiting. A
capacitor on the RES pin (C
RES
) determines the time the controller will remain in low power standby mode before
automatically restarting. A 10µA current source charges the RES pin capacitor to the 1.25V threshold which
restarts the overloaded channel. The two regulator channels operate independently. One channel may operate
normally while the other is in the hiccup mode overload protection. The hiccup mode commences when either
channel experiences 256 consecutive PWM cycles with cycle-by-cycle current limiting. If that occurs, the
overloaded channel will turn off and remain off for the duration of the RES pin timer.
The hiccup mode current limiting function can be disabled. The RES configuration is latched during initial power-
up when UVLO is above 1.25V and VCC1 and VCC2 are above their UV thresholds, determining hiccup or non-
hiccup current limiting. If the RES pin is tied to VCC at initial power-on, hiccup current limit is disabled.
Soft-Start
The soft-start feature allows the regulator to gradually reach the steady state operating point, thus reducing start-
up stresses and surges. The LM5119 will regulate the FB pin to the SS pin voltage or the internal 0.8V reference,
whichever is lower. At the beginning of the soft-start sequence when SS = 0V, the internal 10µA soft-start current
source gradually increases the voltage on an external soft-start capacitor (C
SS
) connected to the SS pin resulting
in a gradual rise of the FB and output voltages.
Either regulator channel of the LM5119 can be disabled by pulling the corresponding SS pin to AGND.
Diode Emulation
A fully synchronous buck regulator implemented with a free-wheel MOSFET rather than a diode has the
capability to sink current from the output in certain conditions such as light load, over-voltage or pre-bias startup.
The LM5119 provides a diode emulation feature that can be enabled to prevent reverse (drain to source) current
flow in the low side free-wheel MOSFET. When configured for diode emulation, the low side MOSFET is disabled
when reverse current flow is detected. The benefit of this configuration is lower power loss at no load or light load
conditions and the ability to turn on into a pre-biased output without discharging the output. The diode emulation
mode allows for start-up into pre-biased loads, since it prevents reverse current flow as the soft-start capacitor
charges to the regulation level during startup. The negative effect of diode emulation is degraded light load
transient response times. Enabling the diode emulation feature is recommended and allows discontinuous
conduction operation. The diode emulation feature is configured with the DEMB pin. To enable diode emulation,
connect the DEMB pin to ground or leave the pin floating. If continuous conduction operation is desired, the
DEMB pin should be tied to either VCC1 or VCC2.
Copyright © 2010–2013, Texas Instruments Incorporated Submit Documentation Feedback 15
Product Folder Links: LM5119/LM5119Q