LM5576 datasheet(12/30 Pages) TI

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RAMP

VCC

CRAMP

RRAMP

LM5576, LM5576-Q1

SNVS447I – JANUARY 2007 – REVISED APRIL 2013

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Proper selection of the RAMP capacitor depends upon the selected value of the output inductor. The value of

CRAMP can be selected from: CRAMP = L x 10

-5, where L is the value of the output inductor in Henrys. With this

value, the scale factor of the emulated current ramp will be approximately equal to the scale factor of the DC

level sample and hold ( 0.5 V / A). The CRAMP capacitor should be located very close to the device and

connected directly to the pins of the IC (RAMP and AGND).

For duty cycles greater than 50%, peak current mode control circuits are subject to sub-harmonic oscillation.

Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow pulses at the switch

node. Adding a fixed slope voltage ramp (slope compensation) to the current sense signal prevents this

oscillation. The 25µA of offset current provided from the emulated current source adds some fixed slope to the

ramp signal. In some high output voltage, high duty cycle applications, additional slope may be required. In these

applications, a pull-up resistor may be added between the VCC and RAMP pins to increase the ramp slope

compensation.

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)

Figure 15. RRAMP to VCC for VOUT > 7.5V

Note that the emulated ramp signal on CRAMP is applied to the current limit comparator as described in the

Current Limit section below. Increasing the ramp slope will result in lower current limit threshold. This can lower

the output current capability of the part to less than 3A in some conditions. The resulting current limit threshold

can be calculated by the following equation:

where

•

VCL = 2.1V

•

gm = 5µA/V

•

Ioffset = 25µA

•

A x Rs = 0.5V/A

•

Vcc = 7V

•

T = switching period

•

D = duty cycle (approximately VOUT / VIN)

•

L = inductor value

•

CRAMP = ramp capacitor value

•

RRAMP = ramp resistor value

(3)

If the recommended CRAMP and RRAMP values are used, then the following simplified equation calculates the

current limit threshold:

(4)

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