SC488 datasheet(15/24 Pages) SEMTECH

15 / 24 page

www.semtech.com

SC488

POWER MANAGEMENT

We will select an inductor value of 1.5μH to reduce the

ripple current, which can be calculated as follows:

)

MIN

(

VBAT

ON_

OUT

)

MIN

(

BAT

)

MIN

(

BAT

RIPPLE_V

and,

)

MAX

(

VBAT

ON_

OUT

)

MAX

(

BAT

)

MAX

(

BAT

RIPPLE_V

For our example:

RIPPLE_VBAT(MIN) = 3.39AP-P and IRIPPLE_VBAT(MAX) = 4.34AP-P

From this we can calculate the minimum inductor current

rating for normal operation:

(MIN)

)

MAX

(

RIPPLE_VBA

)

MAX

(

OUT

)

MIN

(

INDUCTOR

For our example:

INDUCTOR(MIN) = 12.2A(MIN)

Next we will calculate the maximum output capacitor

equivalent series resistance (ESR). This is determined by

calculating the remaining static and transient tolerance

allowances. Then the maximum ESR is the smaller of the

calculated static ESR (R

ESR_ST(MAX)) and transient ESR

ESR_TR(MAX)):

Ohms

)

MAX

(

BAT

RIPPLE

ERR

)

MAX

(

ESR

Where ERR

ST is the static output tolerance and ERRDC is

the DC error. The DC error will be 1% plus the tolerance

of the internal feedback. (Use 2% for external feedback

which is 1% plus another 1% for the external resistors.)

For our example:

ERR

ST = 36mV and, ERRDC = 18mV, therefore,

ESR_ST(MAX) = 8.3mΩ

Ohms

)

MAX

(

RIPPLE_VBA

TRANS

ERR

)

MAX

(

ESR_

where ERR

TR is the transient output tolerance. For this

case, I

TRANS is the load transient of 5A (10A - 5A).

For our example:

ERR

TR = 144mV and ERRDC = 18mV, therefore,

ESR_TR(MAX) = 17.6mΩ for a full 5A load transient.

We will select a value of 6mΩ maximum for our design,

which would be achieved by using two 12mΩ output ca-

pacitors in parallel. Now that we know the output ESR we

can calculate the output ripple voltage:

)

MIN

(

RIPPLE_VBA

ESR

)

MIN

(

RIPPLE_VBA

and,

)

MAX

(

RIPPLE_VBA

ESR

)

MAX

(

RIPPLE_VBA

For our example:

RIPPLE_VBAT(MAX) = 20mVP-P and VRIPPLE_VBAT(MIN) = 26mVP-P

Note that in order for the device to regulate in a controlled

manner, the ripple content at the feedback pin, V

FB, should

be approximately 15mV

P-P at minimum VBAT, and worst case

no smaller than 10mV

P-P. Note that the voltage ripple at

FB is smaller than the voltage ripple at the output capaci-

tor, due to the resistor divider. Also, when using internal

feedback (FB pin tied to 5V or GND), the FB resistor di-

vider is actually inside the IC. If V

RIPPLE_VBAT(MIN) as seen at

the FB point is less than 15mV

P-P - whether internal or ex-

ternal FB is used - the above component values should be

revisited in order to improve this. For our example, since

the internal divider reduces the ripple signal by a factor of

(1.5V/1.8V), the internal FB ripple values are then 17mV

and 22mV, which is above the 15mV minimum.

Application Information (Cont.)

SC488 Datenblatt(PDF) 15 Page - Semtech Corporation

SC488 Datenblatt(HTML) 15 Page - Semtech Corporation

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datenblatt Download

Link URL