SC2447TSTRT datasheet(20/26 Pages) SEMTECH

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POWER MANAGEMENT

SC2447

average current mode control), this current sensing

method is sufficient without time constant matching

requirement.

In the current mode control as implemented in SC2447,

voltage ripple on C

s is required for PWM operation.

fact, the V

Cs AC peak-to-peak voltage ripple (denoted as

∆V

Cs) directly affects 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:

∆V

Cs~ RL δIo.

It is necessary to match the following time constants to

equalize the ripple.

L ≈

For example, L = 1µH and R

L = 1.8mΩ, the time constant

sCs should be set to 555.6µs. If one selects Cs = 33nF,

then R

s = 16.9 kΩ.

Scaling the Current Limit

Over-current is handled differently in the SC2447

depending on the direction of the inductor current. If

the differential sense voltage between CS+ and CS-

exceeds +50mV, the PWM signal (GDH) will go low. The

MOSFET driver will turn off the top MOSFET and turn on

the bottom MOSFET, 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.

In the circuit of Figure 13, the equivalent inductor current

limit is set according to

LMcp

when the load is sourcing current from the converter. If

L = 1.8mΩ, then ILMcp = 27.8A.

The circuit in Figure 14

allows the user to scale the equivalent current limit with

the same R

S in the current sensing network is usually in the range

of 22nF ~ 330nF.

a) When the required current limit I

LM is higher than ILMcp,

s3 is not needed and solve the following three equations

for R

s, Rs1, and Rs2:

)

(

and

Note that R

S2 is made equal to RS//RS1 to reduce effect

of the bias current of the current amplifier in SC2447.

b) When the required current limit I

LM is less than ILMcp,

remove R

s1 and solve the following two equations for Rs

and R

s3:

s2 is then calculated from

−

Cin

Cout

Rload

Vin

vC(t)

iL(t)

Vgs2

Vgs1

ISEN

Rs3

Rs2

Rs1

Figure 14. Scaling the Equivalent Current Limit

Application Information (Cont.)

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