LM5574-Q1

SNOSB22C – OCTOBER 2008 – REVISED OCTOBER 2014

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Feature Description (continued)

An external set-point voltage divider from VIN to GND can be used to set the operational input range of the

regulator. The divider must be designed such that the voltage at the SD pin will be greater than 1.225 V when

Vin is in the desired operating range. The internal 5 µA pull-up current source must be included in calculations of

the external set-point divider. Hysteresis of 0.1 V is included for both the shutdown and standby thresholds. The

SD pin is internally clamped with a 1 k

Ω resistor and an 8 V zener clamp. The voltage at the SD pin should never

exceed 14 V. If the voltage at the SD pin exceeds 8 V, the bias current will increase at a rate of 1 mA/V.

The SD pin can also be used to implement various remote enable / disable functions. Pulling the SD pin below

the 0.7 V threshold totally disables the controller. If the SD pin voltage is above 1.225 V the regulator will be

operational.

7.3.2 Current Limit

The LM5574-Q1 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 2.0 V / A. The emulated ramp signal is applied to the current limit comparator. If the

emulated ramp signal exceeds 1.4 V (0.7 A) 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 & hold

DC level exceeds the 1.4 V 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.

7.3.3 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 10 µA, 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.

When the fault condition is no longer present a new soft-start sequence will commence.

7.3.4 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 180°C, 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.

7.4 Device Functional Modes

7.4.1 High Voltage Start-Up Regulator

for the PWM controller and boot-strap MOSFET gate driver. The input pin (VIN) can be connected directly to the

range of 6 V to 75 V is achieved through the use of this dual mode regulator.

V and the SD pin is greater than 1.225 V, the output switch is enabled and a soft-start sequence begins. The

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