LM3477AMMXSLASHNOPB datasheet(17/35 Pages) TI

17 / 35 page

IHYS =

MAX (VHYS - 50 x 10

-6 x R

SL x DMAX, 0)

RSN

(A)

RSN(MAX) =

VCL(0)(MIN) - DMAX (VCL(0)(MIN) - (VCL(100)(MIN) - 50 x 10

-6

x RSL))

VOUT(1-DMAX)

2 x L x fS

IOUT +

1.15 x IOUT

VCL(0)(MIN) - DMAX (VCL(0)(MIN) - (VCL(100)(MIN) - 50 x 10

-6

x RSL))

DUTY CYCLE (%)

100

200

100

VCL(0)

VSEN

VSEN(PEAK)

VCL(D)

VSL

VCL(100)

IHYS =

0.032

0.02

= 1.6A, LM3477

0.011

0.02

0.55A, LM3477A

0.032

RSN

(A), LM3477

0.011

RSN

(A), LM3477A

IHYS =

VHYS

RSN

LM3477

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SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

The hysteretic threshold is derived in a similar manner, the only difference being that VSEN(peak) is compared

VC(min) (VHYS). Notice that VHYS does not vary with the duty cycle. The hysteretic threshold is predetermined by

the selection of RSN above. The hysteretic threshold is:

(13)

Continuing with the example above,

(14)

If the peak switch current decreases below this threshold, the LM3477/A will operate in hysteretic mode (see

OVER VOLTAGE PROTECTION). In some designs, it will be desired to use RSL so that lower valued inductors

can be used (see DEFAULT/ADJUSTABLE SLOPE COMPENSATION and POWER INDUCTOR SECTION).

Using RSL will lower the current limit and the hysteretic threshold. See Figure 27. RSL effectively adds an

additional slope to the existing slope of the VC waveform.

Figure 27. Current Limit and Hysteretic Threshold vs Duty Cycle with RSL

When RSL is used, the following equations apply:

(15)

(16)

where MAX(VHYS − 50x10

-6 x R

SL x DMAX, 0) is the smaller of the two values in the parenthesis and VHYS is

0.032V and 0.011V for the LM3477 and LM3477A, respectively. RSL can be used creatively to intentionally lower

the hysteretic threshold, allowing for better performance at lower loads. However, when RSL is used, there may

be a minimum load requirement (see START-UP/SOFT-START).

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