Datasheet

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DEMB

1.215V

SS Latch

COMPARATOR

DIODE EMULATION

40k

LM25116

www.ti.com

SNVS509D –APRIL 2007–REVISED FEBRUARY 2013

Soft-Start and Diode Emulation

The soft-start feature allows the regulator to gradually reach the initial steady state operating point, thus reducing

start-up stresses and surges. The LM25116 will regulate the FB pin to the SS pin voltage or the internal 1.215V

reference, whichever is lower. At the beginning of the soft-start sequence when SS = 0V, the internal 10 µA soft-

start current source gradually increases the voltage of an external soft-start capacitor (C

) connected to the SS

pin resulting in a gradual rise of FB and the output voltage.

Figure 34. Diode Emulation Control

During this initial charging of C

to the internal reference voltage, the LM25116 will force diode emulation. That

is, the low-side MOSFET will turn off for the remainder of a cycle if the sensed inductor current becomes

negative. The inductor current is sensed by monitoring the voltage between SW and DEMB. As the SS capacitor

continues to charge beyond 1.215V to 3V, the DEMB bias current will increase from 0 µA up to 40 µA. With the

use of an external DEMB resistor (R

DEMB

), the current sense threshold for diode emulation will increase resulting

in the gradual transition to synchronous operation. Forcing diode emulation during soft-start allows the LM25116

to start up into a pre-biased output without unnecessarily discharging the output capacitor. Full synchronous

operation is obtained if the DEMB pin is always biased to a higher potential than the SW pin when LO is high.

DEMB

= 10 kΩ will bias the DEMB pin to 0.45V minimum, which is adequate for most applications. The DEMB

bias potential should always be kept below 2V. At very light loads with larger values of output inductance and

MOSFET capacitance, the switch voltage may fall slowly. If the SW voltage does not fall below the DEMB

threshold before the end of the HO fall to LO rise dead-time, switching will default to diode emulation mode.

When R

DEMB

= 0Ω, the LM25116 will always run in diode emulation.

Once SS charges to 3V the SS latch is set, increasing the DEMB bias current to 65 µA. An amplifier is enabled

that regulates SS to 160 mV above the FB voltage. This feature can prevent overshoot of the output voltage in

the event the output voltage momentarily dips out of regulation. When a fault is detected (VCC under-voltage,

UVLO pin < 1.215, or EN = 0V) the soft-start capacitor is discharged. Once the fault condition is no longer

present, a new soft-start sequence begins.

HO Output

The LM25116 contains a high current, high-side driver and associated high voltage level shift. This gate driver

circuit works in conjunction with an external diode and bootstrap capacitor. A 1 µF 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 -0.5V 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 450 ns to ensure that the bootstrap capacitor is recharged.

The LO and HO outputs are controlled with an adaptive deadtime methodology which insures that both outputs

are never enabled at the same time. When the controller commands HO to be enabled, the adaptive block first

disables LO and waits for the LO voltage to drop below approximately 25% of VCC. HO is then enabled after a

small delay. Similarly, when HO turns off, LO waits until the SW voltage has fallen to ½ of VCC. LO is then

enabled after a small delay. In the event that SW does not fall within approximately 150 ns, LO is asserted high.

This methodology insures adequate dead-time for appropriately sized MOSFETs.

Product Folder Links: LM25116