13

© 2004 Semtech Corp.

www.semtech.com

SC4510

POWER MANAGEMENT

In the application circuit, R

value of the sensed current is not exact for a number of

reasons. Though the Phase Node PH and Virtual Phase

Node VPN voltages are assumed to be identical, there will

be some offset between them which adds to the average

value of the current feedback signal. This is particularly

true during the switching transitions where rise and fall

times of the true Phase Node are dependent on the power

MOSFET characteristics. The peak value of the signal also

includes the ripple current

current. In addition, the signal level is subject to variations

with respect to input and PVCC voltages. The difference

between sensed current magnitude and the actual current

gets more pronounced for low output voltages where the

operating duty is also low.

Given these differences, some amount of scaling the

current sense signal is required to adjust the current limit

in most applications. This can be accomplished easily by

simple resistor networks and the possible configurations

are shown in Fig.3.

a) When the required current limit value I

I

R

Fig 3). R

sense amplifier inside SC4510.

Q1

Q2

Cin

Cout

RL

Rload

Rs

Vin

Vo

Cs

vC(t)

L

iL(t)

Q3

Q4

Vgs2

Vgs1

VPN

Vbe4

Vbe3

PN

-

+

ISEN

1

2

Rs3

Rs2

Rs1

eq

s

seq

R

L

C

R

=

In peak current mode control as in SC4510, the voltage

ripple on C

to-peak value of the voltage ripple across V

∆V

operation. In general, smaller

∆V

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

Where

The prerequisite for such relation is the so called time

constant matching condition

When Rds1 = Rds2, the above approximations become

precise equalities. For the example in the Application Circuit

shown on p9 the inductor value is 1 uH,

RL= 1.4 m

Ω , Rds1 = 11 mΩ and Rds2 = 3 mΩ

Since the operating duty ratio is very small the effective

Rds is determined mostly by Rds2. The time constant R

should be set close to 0.2 mS. Since the effective value of

R

C

CURRENT LIMIT SCALING

In SC4510, the current limiting is performed on cycle-by-

cycle basis. When the voltage difference between CS+ and

CS- exceeds 65 mV, the top MOSFET duty ratio is clipped

in order to limit the output source current. Similarly, when

the voltage difference between CS- and CS+ exceeds 113

mV, the bottom MOSFET duty ratio is clipped in order to

limit the sink current. For the configuration in Fig.1, the

convertor output current limit is set around

for the current sourcing mode and

for the current sinking mode.

Application Information (Contd.)

.

s

s

eq

C

R

R

L

≈

,

75

eq

LMcp

R

mV

I

=

eq

LMcn

R

mV

I

110

−

=

Fig.3 Current limit scaling.