SC1406GCTSTR datasheet(16/28 Pages) SEMTECH

16 / 28 page

ã 2000 Semtech Corp.

www.semtech.com

POWER MANAGEMENT

SC1406G

Rearranging equation 1), we select the minimum inductor value.

This value of inductance is required up to maximum load.

Inductors with a swinging choke characteristic, where the zero

current value of inductance is much less than the full load

current inductance can be used, as long as the above restriction

is met. A value of 1.5uH is used to allow tolerances. Then, the

worst-case (low input voltage) response time (the time for the

current to reach the new transient value) is:

Add ~100ns for the SC1405/06 response. Since the shaded

area is triangular, the total charge taken out of the capacitor =

(DI ? Dt) / 2. Q = C ? DV = (DI ? Dt) / 2, therefore;

This condition applies only to the positive transient. For

negative load steps, the capacitance also has to be large

enough to absorb the energy in the inductance. Since:

Using Panasonic SP-Caps, the EEFUE0E221R is 220uF at 2.5V

with 15mW ESR. Two are sufficient to meet ESR requirements,

but three are required to meet the capacitance requirement,

when tolerances are considered. The Sanyo POSCAP 4TPC150

is a 150uF capacitor with a maximum ESR specification of

45mW; six are required to meet the ESR requirement. The

resulting 900mF exceeds the capacitance requirement.

Other Component Selection:

As a compromise between current sensing accuracy, efficiency,

and availability, a current sense resistor of 3mW is used. The

power dissipation is I2xR, or 555mW, so choose a 1W or larger

resistor for design margin.

The connections to CMP, CMPREF, CL, and CLREF (pins 25

through 28) are two differential pairs connected to the current

sense resistor. In order to cancel out common-mode noise,

resistor pairs R3-R4 and R5-R6 should be equal. For the time

being, assume R3=R4=R5=R6=1kW. These values may be

adjusted later if required.

The current limit is a function of peak current and should be set

at about 125% of the peak to allow for some overshoot of

inductor current during transients. For L=1.5uH, and

F=300kHz:

So, set the current limit at approximately 18.5A. Using equation

10):

Using equation 6) we calculate R

DAC:

The offset resistor, R22, is calculated from equation 8):

Equation 5) is used to calculate R1 by calculating a new value of

HYS which accounts for the IMVP and offset dividers, and then

plugging the resulting value into equation 3).

LMIN

dMIN

VINMAX VDAC

−

() ESR

MAX

RCS

()

⋅

FS VRIPPLE

⋅

LMIN 1.426 10

−

LMIN ICC650MAX ICC650SG

−

()

⋅

VINMIN

VDAC

−

1.936

−

CMINP

ICC650MAX ICC650SG

−

() dT 100 10 9

−

⋅

sec

⋅

()

⋅

2VIMVP

⋅

1.186

−

CMINN

LMIN

ICC650MAX

ICC650SG

−





VCC650MAXTRANS

VFL

−





⋅

CMINN 4.045 10

−

IPEAK

ICC650MAX

VINMAX VDAC

−

() d

MIN

⋅

2LMIN

⋅

IPEAK 15.327 A

ICLMAX IPEAK 1.25

⋅

ICLMAX 19.159A

RCLSET

3VREF

⋅

RCLOH

⋅

RCS ICLMAX

⋅

RCLSET

8.873

×Ω

RDAC

VIMVP ICC650MAX RCS

⋅

−

() R

CORE

⋅

ICC650MAX RCS

⋅

RDAC 1.397 10

×Ω

ROFF

ROH

VNL RDAC

⋅

VDAC

VNL

VDAC

−











ROFF 1.068 10

×Ω

VHYS

VRIPPLE RCORE ROFF

⋅

ROH RDAC

⋅

−

()

⋅

ESR

RCS RCORE

⋅

ROFF

ROH

()

RCORE ROFF

⋅

RDAC ROH

⋅

−

⋅













2 ESR

⋅

RCORE

⋅

ROFF

ROH

()

⋅





⋅

RHYS

VREF ROH

⋅

VHYS

⋅

V HYS

0.027 V

R HYS

1.281

×Ω

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