RSN(MAX)=

0.135 - 0.6 (0.135 - .025)

1.15 (3)

= 0.02:

RSN(MAX) =

VCL(0)(MIN) - DMAX (VCL(0)(MIN) - VCL(100)(MIN))

1.15 x IOUT(MAX)

|

VCL(0)(MIN) - DMAX (VCL(0)(MIN) - VCL(100)(MIN))

VOUT(1-DMAX)

2 x L x fS

IOUT(MAX) +

VSEN(PEAK) =

) (V), RSL = 0

VOUT(1-DMAX)

2 x L x fS

RSN(IOUT(MAX) +

RSN x IOUT(MAX)(1+ 0.15) (V)

|

xx

xx

xx

xx

DUTY CYCLE (%)

0

20

40

60

80

0

100

200

100

VCL(0)

VCL(D)

32

11

VSEN

VHYS(LM3477)

VHYS(LM3477A)

VSEN(PEAK)

VCL(100)

LM3477

SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

www.ti.com

PROGRAMMING THE CURRENT LIMIT/HYSTERETIC THRESHOLD

Definitions

Current Limit: The current limit is the point at which the LM3477/A begins to limit the peak switch current. The

current limit in the LM3477/A varies with duty cycle, which is a function of the VIN − VOUT differential.

Hysteric Threshold: Hysteretic threshold is the current at which the LM3477/A enters the hysteretic mode of

operation (see OVER VOLTAGE PROTECTION). The hysteretic threshold is with respect to the peak switch

current.

SETTING CURRENT LIMIT AND HYSTERETIC THRESHOLD

The adjustable current limit of the LM3477/A is set by the sense resistor RSN. The voltage across RSN is

compared to an internal control voltage VC. The onset of current limiting is when VSEN(peak) equals VC(max), or VCL.

VSEN is defined here as the differential voltage from the VIN pin to the ISEN pin. VCL decreases as the duty cycle

increases, as shown in Figure 26. Therefore, it is important to know both VSEN(peak) and VCL(min) at the maximum

operating duty cycle, or lowest VIN condition.

Figure 26. Current Limit and Hysteretic Threshold vs Duty Cycle

(8)

VCL(MIN) = VCL(0)(MIN) − D(MAX) (VCL(0)(MIN) − VCL(100)(MIN))

(9)

where DMAX is the duty cycle at the lowest VIN condition.

To avoid current limit,

VSEN(peak) < VCL(MIN)

(10)

Therefore,

(11)

Example: VIN(MIN) = 4.5V, VOUT = 2.5V, IOUT(MAX) = 3A

(12)

16

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