LM3477AMM datasheet(14/23 Pages) NSC

14 / 23 page

Design Section (Continued)

This is the fraction of the switching period that the switch is

on. The switch is off for the remainder of the period. This

fraction is expressed as:

D’ = 1−D

The LM3477/A has limits for the maximum and minimum

duty cycle (see ELECTRICAL CHARACHTERISTICS). The

maximum duty cycle of 93% (typical) will limit how low the

input voltage may drop while maintaining a regulated output

voltage (the dropout voltage). In situations where a very low

dropout voltage is required, it is necessary to include V

D,VQ

and V

SN losses in the maximum duty cycle calculation.

Voltage drops in the inductor will lower the dropout voltage

as well.

The minimum duty cycle of the LM3477/A corresponds to the

minimum on time, or blank out time (see ELECTRICAL

CHARACHTERISTICS).

MIN =TMIN* fs

This will not limit how high the input voltage can rise, how-

ever the LM3477/A will operate in hysteretic mode once the

operating duty cycle decreases to the minimum duty cycle.

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 cur-

rent limit in the LM3477/A varies with duty cycle, which is a

function of the V

IN −VOUT differential.

Hysteric Threshold: Hysteretic threshold is the current at

which the LM3477/A enters the hysteretic mode of operation

(see OVER VOLTAGE PROTECTION section). The hyster-

etic 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 R

SN. The voltage across RSN is compared to

an internal control voltage V

C. The onset of current limiting is

when V

SEN(peak) equals VC(max),orVCL.VSEN is defined

here as the differential voltage from the V

IN pin to the ISEN

pin. V

CL decreases as the duty cycle increases, as shown in

Figure 7. Therefore, it is important to know both V

SEN(peak)

and V

CL(min) at the maximum operating duty cycle, or lowest

IN condition.

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

where D

MAX is the duty cycle at the lowest VIN condition.

To avoid current limit,

SEN(peak) < VCL(MIN)

Therefore,

200033K3

FIGURE 7. Current Limit and Hysteretic Threshold vs

Duty Cycle

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