VinMIN =

Vout + VD

1 - Fs x 500 ns

RAMP

VCC

CRAMP

RRAMP

LM5576Q0

SNOSB24B – OCTOBER 2008 – REVISED APRIL 2013

www.ti.com

For VOUT > 7.5V:

Calculate optimal slope current, IOS = VOUT x 5µA/V.

For example, at VOUT = 10V, IOS = 50µA.

Install a resistor from the RAMP pin to VCC:

RRAMP = VCC / (IOS - 25µA)

(4)

Figure 15. RRAMP to VCC for VOUT > 7.5V

MAXIMUM DUTY CYCLE / INPUT DROP-OUT VOLTAGE

There is a forced off-time of 500ns implemented each cycle to ensure sufficient time for the diode current to be

sampled. This forced off-time limits the maximum duty cycle of the buck switch. The maximum duty cycle will

vary with the operating frequency.

DMAX = 1 - Fs x 500ns

where

•

Fs is the oscillator frequency.

(5)

Limiting the maximum duty cycle will raise the input dropout voltage. The input dropout voltage is the lowest input

voltage required to maintain regulation of the output voltage. An approximation of the input dropout voltage is:

where

•

VD is the voltage drop across the re-circulatory diode.

(6)

Operating at high switching frequency raises the minimum input voltage necessary to maintain regulation.

CURRENT LIMIT

The LM5576Q0 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 2.1V (4.2A) 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 2.1V 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.

12

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