SC2447 查看數據表（PDF） - Semtech Corporation

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SC2447

Dual-Phase, Single or Dual Output Synchronous Step-Down Controller

Semtech Corporation 

SC2447

POWER MANAGEMENT

Application Information (Cont.)

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 is required for PWM operation. In

s

fact, the V AC peak-to-peak voltage ripple (denoted as

Cs

∆VCs) directly affects the signal-to-noise ratio of the PWM

operation. In general, smaller ∆VCs leads to lower signal-

to-noise ratio and more noise sensitive operation. Larger

∆V leads to more circuit (power stage) parameter

Cs

sensitive operation. A good engineering compromise is:

of 22nF ~ 330nF.

a) When the required current limit I is higher than I ,

LM

LMcp

Rs3 is not needed and solve the following three equations

for Rs, Rs1, and Rs2:

(Rs

//

Rs1 )

Cs

=

L

RL

,

I LM RL

Rs1

Rs + Rs1

=

50mV ,

∆V ~ R δI .

Cs

Lo

and

Rs 2 = Rs // Rs1 .

It is necessary to match the following time constants to Note that RS2 is made equal to RS//RS1 to reduce effect

equalize the ripple.

of the bias current of the current amplifier in SC2447.

L

RL

≈

RsCs .

For example, L = 1µH and R = 1.8mΩ, the time constant

L

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

then Rs = 16.9 kΩ.

b) When the required current limit ILM is less than ILMcp,

remove Rs1 and solve the following two equations for Rs

and Rs3:

RsCs

=

L

RL

,

Scaling the Current Limit

Over-current is handled differently in the SC2447

I LM

RL

+

Rs

Rs3

VO

=

50mV ,

depending on the direction of the inductor current. If

the differential sense voltage between CS+ and CS-

Rs2 is then calculated from

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

Rs 2

=

Rs 3 Rs

Rs3 − Rs

.

+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

I LMcp

=

50mV

RL

,

Vin

Cin

Q1

Vgs1

PN

iL(t) L

Rs

Q2

Vgs2

RL

Rs1

Cs

vC(t)

Vo

Cout Rload

when the load is sourcing current from the converter. If

RL = 1.8mΩ, then ILMcp = 27.8A. The circuit in Figure 14

allows the user to scale the equivalent current limit with

the same RL.

ISEN

+1

-2

Rs2

Rs3

CS in the current sensing network is usually in the range

Figure 14. Scaling the Equivalent Current Limit

© 2006 Semtech Corp.

20

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