LM5005
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SNVS397D –SEPTEMBER 2005–REVISED MARCH 2013
Figure 15. R
RAMP
to V
CC
for V
OUT
> 7.5V
Current Limit
The LM5005 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 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 LM5005 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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