SC488ML 查看數據表（PDF） - Semtech Corporation

零件编号

产品描述 (功能)

生产厂家

SC488ML

Complete DDR1/2/3 Memory Power Supply

Semtech Corporation 

SC488

POWER MANAGEMENT

Application Information (Cont.)

We will select an inductor value of 1.5μH to reduce the RESR_ST(MAX) = 8.3mΩ

ripple current, which can be calculated as follows:

I RIPPLE_VBAT(MIN)

¨©§ VBAT (MIN)  V OUT ¸¹· x

tON_ VBAT ( MIN )

L

AP P

R ESR_ TR (MAX)

¨¨©§I

¨©§ ERRTR  ERRDC ¸¹·

TRANS



IRIPPLE_VBAT(

2

MAX

)

¸·

¸¹

Ohms

and,

I RIPPLE_VBAT(MAX )

¨©§ VBAT (MAX )  V OUT ¸¹· x

tON_ VBAT ( MAX )

L

AP P

where ERRTR is the transient output tolerance. For this

case, ITRANS is the load transient of 5A (10A - 5A).

For our example:

For our example:

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

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

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

From this we can calculate the minimum inductor current

rating for normal operation:

I INDUCTOR ( MIN )

IOUT (MAX ) 

I RIPPLE_VBAT ( MAX )

2

A (MIN)

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:

For our example:

IINDUCTOR(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 (RESR_ST(MAX)) and transient ESR

(RESR_TR(MAX)):

  RESR _ ST (MAX )

ERR ST  ERRDC

x2

Ohms

I RIPPLE _ V BAT (MAX)

Where ERRST 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:

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

V RIPPLE_VBAT ( MIN) RESR x IRIPPLE_VBAT (MIN)VPP

and,

V RIPPLE_VBAT ( MAX ) RESR x IRIPPLE_VBAT (MAX )VPP

For our example:

VRIPPLE_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, VFB, should

be approximately 15mVP-P at minimum VBAT, and worst case

no smaller than 10mVP-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 VRIPPLE_VBAT(MIN) as seen at

the FB point is less than 15mVP-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.

© 2006 Semtech Corp.

15

www.semtech.com

Share Link: