LM25005MH datasheet(14/21 Pages) NSC

14 / 21 page

Current Limit

The LM25005 contains a unique current monitoring scheme

for control and over-current protection. When set correctly,

the emulated current sense signal provides a signal which is

proportional to the buck switch current with a scale factor of

0.5 V / A. The emulated ramp signal is applied to the current

limit comparator. If the emulated ramp signal exceeds 1.75V

(3.5A) the present current cycle is terminated (cycle-by-cycle

current limiting). In applications with small output inductance

and high input voltage the switch current may overshoot due

to the propagation delay of the current limit comparator. If an

overshoot should occur, the diode current sampling circuit

will detect the excess inductor current during the off-time of

the buck switch. If the Sample and Hold DC Level exceeds

the 1.75V current limit threshold, the buck switch will be

disabled and skip pulses until the diode current sampling

circuit detects the inductor current has decayed below the

current limit threshold. This approach prevents current run-

away conditions due to propagation delays or inductor satu-

ration since the inductor current is forced to decay following

any current overshoot.

Soft-Start

The soft-start feature allows the regulator to gradually reach

the initial steady state operating point, thus reducing start-up

stresses and surges. The internal soft-start current source,

set to 10µA, gradually increases the voltage of an external

soft-start capacitor connected to the SS pin. The soft-start

capacitor voltage is connected to the reference input of the

error amplifier. Various sequencing and tracking schemes

can be implemented using external circuits that limit or

clamp the voltage level of the SS pin.

In the event a fault is detected (over-temperature, Vcc

UVLO, SD) the soft-start capacitor will be discharged. When

the fault condition is no longer present a new soft-start

sequence will commence.

Boost Pin

The LM25005 integrates an N-Channel buck switch and

associated floating high voltage level shift / gate driver. This

gate driver circuit works in conjunction with an internal diode

and an external bootstrap capacitor. A 0.022µF ceramic

capacitor, connected with short traces between the BST pin

and SW pin, is recommended. During the off-time of the

buck switch, the SW pin voltage is approximately - 0.5V and

the bootstrap capacitor is charged from Vcc through the

internal bootstrap diode. When operating with a high PWM

duty cycle, the buck switch will be forced off each cycle for

500ns to ensure that the bootstrap capacitor is recharged.

Under very light load conditions or when the output voltage is

pre-charged, the SW voltage will not remain low during the

off-time of the buck switch. If the inductor current falls to zero

and the SW pin rises, the bootstrap capacitor will not receive

sufficient voltage to operate the buck switch gate driver. For

these applications, the PRE pin can be connected to the SW

pin to pre-charge the bootstrap capacitor. The internal pre-

charge MOSFET and diode connected between the PRE pin

and PGND turns on each cycle for 250ns just prior to the

onset of a new switching cycle. If the SW pin is at a normal

negative voltage level (continuous conduction mode), then

no current will flow through the pre-charge MOSFET/diode.

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the

integrated circuit in the event the maximum junction tem-

perature is exceeded. When activated, typically at 165 de-

grees Celsius, the controller is forced into a low power reset

state, disabling the output driver and the bias regulator. This

feature is provided to prevent catastrophic failures from ac-

cidental device overheating.

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