LM5010ASDXSLASHNOPB datasheet(16/25 Pages) TI

16 / 25 page

VOUT

LM5010A

VOUT

Cff

LM5010A

VOUT

LM5010A

LM5010A-Q1

SNVS376E – OCTOBER 2005 – REVISED FEBRUARY 2013

www.ti.com

Figure 15. Low Ripple Output

Where the circuit of Figure 15 is not suitable, the circuits of Figure 16 or Figure 17 can be used.

Figure 16. Low Output Ripple Using a Feed-Forward Capacitor

In Figure 16, Cff is added across R1 to AC-couple the ripple at VOUT directly to the FB pin. This allows the ripple

at VOUT to be reduced, in some cases considerably, by reducing R3. In the circuit of Figure 12, the ripple at VOUT

ranged from 50 mVp-p at VIN = 6V to 320 mVp-p at VIN = 60V. By adding a 1000 pF capacitor at Cff and

reducing R3 to 0.75

Ω, the VOUT ripple was reduced by 50%, ranging from 25 mVp-p to 160 mVp-p.

Figure 17. Low Output Ripple Using Ripple Injection

To reduce VOUT ripple further, the circuit of Figure 17 can be used. R3 has been removed, and the output ripple

amplitude is determined by C2’s ESR and the inductor ripple current. RA and CA are chosen to generate a 40-50

mVp-p sawtooth at their junction, and that voltage is AC-coupled to the FB pin via CB. In selecting RA and CA,

VOUT is considered a virtual ground as the SW pin switches between VIN and -1V. Since the on-time at SW varies

inversely with VIN, the waveform amplitude at the RA/CA junction is relatively constant. R1 and R2 must typically

be increased to more than 10k each to not significantly attenuate the signal provided to FB through CB. Typical

values for the additional components are RA = 200k, CA = 680 pF, and CB = 0.01 µF.

INCREASING THE CURRENT LIMIT THRESHOLD

The current limit threshold is nominally 1.25A, with a minimum value of 1.0A. If, at maximum load current, the

lower peak of the inductor current (IPK-in Figure 11) exceeds 1.0A, resistor RCL must be added between SGND and

ISEN to increase the current limit threshold to equal or exceed that lower peak current. This resistor diverts some

of the recirculating current from the internal sense resistor so that a higher current level is needed to switch the

internal current limit comparator. IPK-is calculated from:

Submit Documentation Feedback

Product Folder Links: LM5010A LM5010A-Q1

LM5010ASDX/NOPB Datasheet(PDF) 16 Page - Texas Instruments