SC2446AEVB datasheet(15/27 Pages) SEMTECH

15 / 27 page

 2005 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC2446A

It is noted that the DC value of V

Cs is independent of the

value of L, R

s and Cs. This means that, if only the average

load current information is needed (such as in average

current mode control), this current sensing method is

effective without time constant matching requirement.

In the current mode control as implemented in SC2446A,

the voltage ripple on C

s is critical for PWM operation. In

fact, the AC voltage ripple peak-to-peak value of V

(denoted as

∆V

Cs) directly effects the signal-to-noise ratio

of the PWM operation. In general, smaller

∆V

Cs leads to

lower signal-to-noise ratio and more noise sensitive

operation. Larger

∆V

Cs leads to more circuit (power stage)

parameter sensitive operation. A good engineering

compromise is to make

∆V

Cs~ RL δIo.

The pre-requisite for such relation is the so called time

constant matching condition

≈

For an example of application circuit, L = 1µH,

RL = 1.8m

Ω, the time constant R

sCs should be set as

555.6µs. If one selects C

s = 33nF, then Rs = 16.9 kΩ.

Scaling the Current Limit

Over-current is handled differently in the SC2446A

depending on the direction of the inductor current. If the

differential sense voltage between CS+ and CS- exceeds

+50mV, the top MOSFET will be turned off and the bottom

MOSFET will be turned on to limit the inductor current.

This +50mV is the cycle-by-cycle peak current limit when

the load is drawing current from the converter. There is

no cycle-by-cycle current limit when the inductor current

flows in the reverse direction. If the voltage between

CS1+ and CS- falls below -75mV, the controller will

undergo overload shutdown and time-out with both the

top and the bottom MOSFETs shut off. (See the section

Overload Protection and Hiccup).

In the circuit of Figure 11, the equivalent inductor current

limits are set according to

LMcp =

when the load is sourcing current from the converter and

LMcn

−

when the load is forcing current back to the input power

source. If R

L = 1.8mΩ, then ILM = 27.8 / -41.7A. The circuit

in Figure 11 allows the user to scale the equivalent

current limit with the same RL.

In the following design steps, the capacitor C

S in the

current sensing part is commonly selected in the range

of 22nF ~ 100nF.

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Vgs2

Vgs1

ISEN

Rs3

Rs2

Rs1

Figure 11. Scaling the equivalent current limit.

a) When the required current limit value I

LM is greater

than I

LMcp, one just needs to remove Rs3, and solve the

following equations

)

(

and

for

and

Application Information (Cont.)

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