RAMP

VCC

CRAMP

RRAMP

LM25005

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SNVS411C – JANUARY 2006 – REVISED MARCH 2013

Figure 14. RRAMP to VCC for VOUT > 7.5V

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 runaway conditions due to propagation delays or inductor saturation 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

temperature is exceeded. When activated, typically at 165 degrees 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 accidental device overheating.

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