8

 2007 Semtech Corp.

www.semtech.com

SC4505

POWER MANAGEMENT

1.5A

2

1

81%

S

µ

1

H

µ

1.5

2.7V

0.8A

I

PK

=

×

×

×









+

=

When using tiny inductors for size-sensitive applications,

only a few limited selections are available that fulfill this

need, and all are within the low

µH range of inductance.

We select a Coiltronics 1.5

µH SD3114 series, presently

the only 3mmx3mmx1.5mm inductor that can provide

the required current in the microminiature size required

in space-constrained applications.

Table 4 shows a list of several low profile inductor

manufacturers. Please consult the manufacturers for

detailed information on their entire selection of power

inductors.

Output Capacitor Selection

The next task in SC4505 design is targeting the proper

amount of ripple voltage due to the constant-current LED

loads. The two error amplifiers that control the PWM

converter sense the delta between requested current

and actual current in each output current regulator. On

a cycle-by-cycle basis, a small amount of output ripple

ensures good sensing and tight regulation, while the

output current regulators keep each LED current at a

fixed value. Overall, this allows usage of small output

caps while ensuring precision LED current regulation.

Although the mechanics of regulation and frequency

dependence may be complex, actual selection of output

capacitor can be simplified to two boundary conditions,

minimum output current and maximum output current.

Output capacitor is chosen to keep ripple voltage

between 10mV & 200mV under all loads.

Design example for Backlight=20mA, Flashlight=100mA:

Minimum Load Current: 20mA (Backlight Only)

Maximum Load Current: 120mA (Backlight and Flash)

All other cases (Torch mode only, Flash mode only,

Backlight and Torch) fall within these two boundary

conditions, so they are automatically satisfied by the

selected output capacitor.

Since the load is a constant current, the capacitor

equation













∆

∆

⋅

=

∆

T

V

C

I

can be solved for the output

ripple.

At 1MHz switching frequency and with the assumption

of the worse case analysis (D=0), an even simpler

relationship can be applied:

where C

For worse case analysis, We see that our typical case of

20mA, 120mA can be immediately converted into its

corresponding ripple relationships of:

∆V

where C

∆V

where C

For the example, if 1

µF output capacitor were used, the

20mV/120mV boundary conditions are well within the

suggested guidelines.

Recommended ceramic capacitor manufacturers are

listed in Table 5.

Applications Information

Table 4. Recommended Inductors

D)

-

1

S

µ

1

C

I

V

T

OU

OUT

OUT

(

×

×

=

∆

S

µ

1

C

I

V

T

OU

OUT

OUT

×

=

∆

PART

L

(

µH)

MAX

DCR

(Ω)

MAX

HEIGHT

(mm)

VENDOR

SD3112-1R0

SD3114-1R5

SD3114-4R7

1.0

1.5

4.7

0.069

0.057

0.147

1.2

1.45

1.45

Coiltronics

www.cooperet.com

LQH3C4R7M24

LQH3C100M24

4.7

10

0.260

0.300

2.2

2.2

Murata

www.murada.com

LB2016B4R7

LB2016B100

4.7

6.8

0.250

0.350

1.6

1.6

Taiyo Yuden

www.t-yuden.com

CMD4D06-4R7

CLQ4D10-4R7

CLQ4D10-6R8

4.7

4.7

6.8

0.216

0.162

0.195

0.8

1.2

1.2

Sumida

www.sumida.com

IHLP2525CZ1R5

1.5

3.3

4.7

0.014

0.028

0.037

3.0

Vishay

www.vishay.com

IHLP2525CZ3R3

IHLP2525CZ4R7

3.0

3.0