LM3477AMM datasheet(15/23 Pages) NSC

15 / 23 page

Design Section (Continued)

Example: V

IN(MIN) = 4.5V, VOUT = 2.5V, IOUT(MAX) =3A

The hysteretic threshold is derived in a similar manner, the

only difference being that V

SEN(peak) is compared VC(min)

HYS). Notice that VHYS does not vary with the duty cycle.

The hysteretic threshold is predetermined by the selection of

SN above. The hysteretic threshold is:

Continuing with the example above,

If the peak switch current decreases below this threshold,

the LM3477/A will operate in hysteretic mode (see OVER

VOLTAGE PROTECTION section). In some designs, it will

be desired to use R

SL so that lower valued inductors can be

used (see DEFAULT/ADJUSTABLE SLOPE COMPENSA-

TION section and Inductor section). Using R

SL will lower the

current limit and the hysteretic threshold. See Figure 8.R

effectively adds an additional slope to the existing slope of

the V

C waveform.

When R

SL is used, the following equations apply:

where MIN(V

HYS − 50x10

-6 xR

SL xDMAX, 0) is the smaller of

the two values in the parenthesis and V

HYS is 0.032V and

0.011V for the LM3477 and LM3477A, respectively. R

SL can

be used creatively to intentionally lower the hysteretic

threshold, allowing for better performance at lower loads.

However, when R

SL is used, there may be a minimum load

requirement (see START-UP/SOFT-START section).

Power Inductor Section

The LM3477/A operates at a high switching frequency of

500kHz, which allows the use of small inductors. This is

made apparent in the following set of equations used to

calculate the output voltage ripple.

∆V

OUT(Pk-Pk)

)

∆i

L(Pk-Pk) xRESR (V)

As the switching frequency fs increases, the inductance

required for a given output voltage ripple decreases. The

equations above for

∆V

OUT

and

∆i

provide criteria for

choosing the inductance. The maximum voltage ripple in

steady-state, PWM operation can be controlled by limiting

∆i

L which in turn is set by the inductance value. Alternatively,

one can simply choose

∆i

L as a percentage of the maximum

output current. Clearly, the size of the output capacitor ESR,

ESR, will have an affect on which criteria is used to choose

the inductance. When the ESR is relatively low (less than

100m

Ω), such as in ceramic, OSCON, and some low ESR

tantalum capacitors, it is convenient to choose the induc-

tance based on setting

∆i

L to 30% of Iout(max). If the ESR is

high, then it may be necessary to restrict

∆i

L to a lower value

so that the output voltage ripple is not too high. Generally

speaking, the former suggestion of setting

∆i

L to 30% of

OUT(MAX) is recommended.

The inductance also affects the stability of the converter. The

slopes S

n and Sf in Figure 4 are functions of the inductance,

while the compensation ramp, S

e, is fixed by default. There-

fore if the inductance is too small, the converter may expe-

rience sub-harmonic oscillations. The LM3477/A provides

sufficient internal slope compensation to allow for induc-

tances chosen according to the

∆i

L = 0.3xIOUT guideline in

most cases. Still, one should check to make sure the induc-

tance is not too low before continuing the design process. If

it is found that the selected inductance is too low, a patented

scheme to increase the compensation ramp, S

e, is provided

in the LM3477/A (see DEFAULT/ADJUSTABLE SLOPE

COMPENSATION section). In the calculations that follow, if it

is found that the chosen inductance is too small, R

SL can be

used to increase Se so that the inductance can be used.

In a current mode control architecture, there is an inherent

resonance

half

the

switching

frequency

(see

DEFAULT/ADJUSTABLE SLOPE COMPENSATION sec-

200033K4

FIGURE 8. Current Limit and Hysteretic Threshold vs

Duty Cycle with R

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