Se = fS(VSL + 50 x 10

-6 x R

SL)

V

Ps

(

)

'Se = 50 x 10

-6 x f

S x RSL

V

Ps

(

)

'In=

Sf - Se

Sn + Se

'In-1

LM3477

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SNVS141K – OCTOBER 2000 – REVISED MARCH 2013

The subharmonic oscillation phenomenon is realized when a load excursion is experienced. The way it is

analyzed is to calculate how the inductor current settles after such an excursion. Take for example the case

when the inductor current experiences a step increase in its average current, shown as the dotted line in

Figure 22. In the switching period that the excursion occurs, the inductor current will change by

ΔI0. In the

following switching period, the inductor current will have a difference

ΔI1 from its original starting value. The

original excursion is being propagated each switching cycle. What is desired is to find out if this propagation is

converging or diverging. It is apparent that the difference in the inductor current from one cycle to the next is a

function of Sn, Sf, and Se, as follows:

(1)

Hence, if the quantity (Sf - Se)/(Sn + Se)is greater than 1, the inductor current diverges and subharmonic

oscillations result. Notice that as Se increases, the factor decreases. Also, when the duty cycle is greater than

50%, as the inductance become less, the factor increases.

The LM3477/A internally generates enough slope compensation Se to allow for the use of reasonable

inductances. The height of the compensation slope ramp VSL can be found in the Electrical Characteristics. The

LM3477/A incorporates a patented scheme to increase Se if there is need to use a smaller inductor. With the use

of a single resistor RSL, Se can be increased indefinitely. RSL increases the compensation slope Se by the

amount:

(2)

Therefore,

(3)

When excursions of the inductor current are divergent, the current sensing control loop is unstable and produces

a subharmonic oscillation in the inductor current. This oscillation is viewed as a resonance in the outer voltage

control loop at half the switching frequency. In POWER INDUCTOR SECTION, calculations for minimum

inductance and necessary slope resistance RSL are carried out based on this resonant peaking.

START-UP/SOFT-START

The LM3477/A incorporates an internal soft-start during start-up. The soft-start forces the inductor current to rise

slowly and smoothly as it increases towards the steady-state current. This technique is used to reduce the input

inrush current during soft-start. The soft-start functionality is effective for approximately the first 5ms of start-up.

NOTE

The LM3477/A will not start-up if the output voltage is biased by more than 200mV above

ground.

NOTE

If the slope resistor RSL is used, the hysteretic threshold will be lowered. Therefore, the

LM3477/A may require up to 100mA of pre-load to successfully start up.

SHORT CIRCUIT PROTECTION

When the voltage across the sense resistor (measured as the VIN − ISEN differential voltage) exceeds VSC, short-

circuit current limit gets activated. In the short-circuit protection mode, the external MOSFET is turned off. When

the short is removed, the external MOSFET is turned on after five cycles. The short circuit protection voltage VSC

is specified in the Electrical Characteristics. VSC is lower in the LM3477A than in the LM3477.

SHUTDOWN

The compensation pin (Pin 2) of LM3477/A also functions as a shutdown pin. If a low signal (refer to the

Electrical Characteristics for definition of low signal) appears on the COMP/SD pin, the LM3477/A stops

switching and goes into a low supply current mode. The total supply current of the IC reduces to less than 10µA

under these conditions. Figure 23 shows different implementations of the shutdown function.

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