IPK =

RCL

1.5A x (150 m: + RCL)

+ IOR(MAX)

RCL =

1.0A x 0.11:

IPK- - 1.0A

IPK- = IO(max) -

2

IOR(min)

SW

FB

VOUT

R2

LM5010A

R1

C2

L1

CB

CA

RA

19

LM5010A, LM5010A-Q1

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In Figure 14, 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 11, the ripple at VOUT

ranged from 50 mVp-p at VIN = 6 V to 320 mVp-p at VIN = 60 V. 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 15. Low Output Ripple Using Ripple Injection

To reduce VOUT ripple further, the circuit of Figure 15 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 to

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 –1 V. Since the ON-time at SW

varies inversely with VIN, the waveform amplitude at the RA and 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 = 200 k, CA = 680 pF, and CB = 0.01 µF.

8.2.2.3 Increasing The Current Limit Threshold

The current limit threshold is nominally 1.25 A, with a minimum value of 1 A. If, at maximum load current, the

lower peak of the inductor current (IPK- in Figure 12) exceeds 1 A, resistor RCL must be added between SGND and

ISEN to increase the current limit threshold to be 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. Calculate IPK- with Equation 24.

where

•

IO(max) is the maximum load current

•

IOR(min) is the minimum ripple current calculated using Equation 20

(24)

RCL is calculated from Equation 25.

where

•

0.11

Ω is the minimum value of the internal resistance from SGND to ISEN

(25)

The next smaller standard value resistor must be used for RCL. With the addition of RCL, and when the circuit is in

current limit, the upper peak current out of the SW pin (IPK in Figure 9) can be as high as Equation 26.

where

•

IOR(max) is calculated using Equation 14

(26)

The inductor L1 and diode D1 must be rated for this current. If IPK exceeds 2 A , the inductor value must be

increased to reduce the ripple amplitude. This will necessitate recalculation of IOR(min), IPK-, and RCL.

Increasing the circuit’s current limit will increase power dissipation and the junction temperature within the

LM5010Ax. See Layout Guidelines for guidelines on this issue.