Datasheet

LM25119/25119Q
www.ti.com SNVS680G –AUGUST 2010–REVISED JANUARY 2014
automatically restarting. A 10 µA current source charges the RES pin capacitor to the 1.25 V 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 1.25 V 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 LM25119 will regulate the FB pin to the SS pin voltage or the internal 0.8 V
reference, whichever is lower. At the beginning of the soft-start sequence when SS = 0 V, the internal 10 µA 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 LM25119 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 LM25119 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.
HO and LO Output Drivers
The LM25119 contains a high current, high-side driver and associated high voltage level shift to drive the buck
switch of each regulator channel. This gate driver circuit works in conjunction with an external diode and
bootstrap capacitor. A 0.1 µF or larger ceramic capacitor, connected with short traces between the HB pin and
SW pin, is recommended. During the off-time of the high-side MOSFET, the SW pin voltage is approximately 0 V
and the bootstrap capacitor charges from VCC through the external bootstrap diode. When operating with a high
PWM duty cycle, the buck switch will be forced off each cycle for 320ns to ensure that the bootstrap capacitor is
recharged.
The LO and HO outputs are controlled with an adaptive dead-time methodology which insures that both outputs
are never enabled at the same time. When the controller commands HO to be enabled, the adaptive dead-time
logic first disables LO and waits for the LO voltage to drop. HO is then enabled after a small delay. Similarly, the
LO turn-on is disabled until the HO voltage has discharged. This methodology insures adequate dead-time for
any size MOSFET.
Care should be exercised in selecting an output MOSFET with the appropriate threshold voltage, especially if
VCC is supplied from the regulator output. During startup at low input voltages the MOSFET threshold should be
lower than the 4 V VCC under-voltage lockout threshold. Otherwise, there may be insufficient VCC voltage to
completely turn on the MOSFET as VCC under-voltage lockout is released during startup. If the buck switch
MOSFET gate drive is not sufficient, the regulator may not start or it may hang up momentarily in a high power
dissipation state. This condition can be avoided by selecting a MOSFET with a lower threshold voltage or if VCC
is supplied from an external source higher than the output voltage. If the minimum input voltage programmed by
the UVLO pin resistor divider is above the VCC regulation level, this precaution is of no concern.
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