LX1675CLQ-TR Datasheet LX1675ILQ, LX1675CLQ, LX1675CLQ-TR | P13-P15

LX1675

PRODUCTION DATA SHEET

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 13

Rev. 1.2a, 2006-02-16

WWW.Microsemi .COM

Multiple Output LoadSHARE™ PWM

THEORY OF OPERATION (CONTINUED)

The circuit in Figure 9 sums a current through a 1MΩ resistor

(Rb) offsetting the phase 2 error amplifier to create an imbalance

in the L1 and L2 currents. Although there are many ways to

calculate component values the approach taken here is to pick Ra,

Rb, Rin, Vout, and inductor ESR. A value for the remaining

resistor Rf can then be calculated.

The first decision to be made is the current sharing ratio.

Follow the previous examples to understand the basics of

LoadSHARE. The most common reason to imbalance the

currents in the two phases is because of limitations on the

available power from the input rails for each phase. Use the

available input power and total required output power to

determine the inductor currents for each phase.

All references are to Figure 9

Calculate the voltages V1 and V2.

VoutESR1L Current1L1V +×=

VoutESR2L Current2L2V +×=

Select values for Ra and Rb (Ra is typically 62KΩ ; Rb

is typically 1MΩ)

3) Calculate the offset voltage Vos at the output of the

offset amplifier

()

RbRa

1V2V

2VVos +×

−

−=

⎟

⎠

⎞

⎜

⎝

⎛

4) Calculate the value for Rf

(select a value for Rin typically 5KΩ)

⎟

⎠

⎞

⎜

⎝

⎛

−

1VVout

VoutVos

RinRf

Due to the high impedances in this circuit layout can affect the

actual current ratio by allowing some of the switching waveforms

to couple into the current summing path. It may be necessary to

make some adjustment in Rf after the final layout is evaluated.

Also the equation for Rf requires very accurate numbers for the

voltages to insure an accurate result.

BI-PHASE, LOADSHARE (SERIES RESISTOR METHOD)

A fourth but less desirable way to produce the ratio current

between the two phases is to add a resistor in series with one of the

inductors. This will reduce the current in the inductor that has the

resistor and increase the current in the inductor of the opposite

phase. The example of Figure 7 can be used to determine the

current ratio by adding the value of the series resistor to the ESR

value of the inductor. The added resistance will lower the overall

efficiency

LoadSHARE ERROR SOURCES

With the high DC feedback gain of this second loop, all phase

timing errors, R

DS(On)

mismatch, and voltage differences across the

half bridge drivers are removed from the current sharing accuracy.

The errors in the current sharing accuracy are derived from the

tolerance on the inductor’s ESR and the input offset voltage

specification of the error amplifier. The equivalent circuit is shown

next for an absolute worst case difference of phase currents

between the two inductors.

VOUT

ESR L2

ESR L1

Phase 2

Phase 1

Offset

Error

5mV

Figure 10 – Error Amplitude

Nominal ESR of 6mΩ. ESR ±5%

Max offset Error = 6mV

+5% ESR L1 = 6.3 mΩ

-5% ESR L2 = 5.7 mΩ

1ESRL

V - 1V

A 12 current 1 phase If

OUT

mV75.6106.312V1V

OUT

=××=−

−

mV.618mV61V2V =

A3214.

10x5.7

10x.618

2L ESR

V-2V

current 2 Phase

OUT

===

−

Phase 2 current is 2.32A greater than Phase 1.

Input bias current also contributes to imbalance.

LX1675

PRODUCTION DATA SHEET

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 14

Rev. 1.2a, 2006-02-16

WWW.Microsemi .COM

Multiple Output LoadSHARE™ PWM

APPLICATION NOTE

UTPUT INDUCTOR

The output inductor should be selected to meet the

requirements of the output voltage ripple in steady-state operation

and the inductor current slew-rate during transient. The peak-to-

peak output voltage ripple is:

RIPPLERIPPLE

IESRV ×=

where

ΔI

OUTIN

−

ΔI is the inductor ripple current, L is the output inductor value

and ESR is the Effective Series Resistance of the output

capacitor.

ΔI should typically be in the range of 20% to 40% of the

maximum output current. Higher inductance results in lower

output voltage ripple, allowing slightly higher ESR to satisfy the

transient specification. Higher inductance also slows the inductor

current slew rate in response to the load-current step change, ΔI,

resulting in more output-capacitor voltage droop. When using

electrolytic capacitors, the capacitor voltage droop is usually

negligible, due to the large capacitance

The inductor-current rise and fall times are:

()

OUTIN

RISE

ΔI

−

×=

and

OUT

FALL

ΔI

LT ×=

.The inductance value can be calculated by

ΔI

OUTIN

−

OUTPUT CAPACITOR

The output capacitor is sized to meet ripple and transient

performance specifications. Effective Series Resistance (ESR) is a

critical parameter. When a step load current occurs, the output

voltage will have a step that equals the product of the ESR and the

current step, ΔI. In an advanced microprocessor power supply, the

output capacitor is usually selected for ESR instead of capacitance

or RMS current capability. A capacitor that satisfies the ESR

requirements usually has a larger capacitance and current capability

than strictly needed. The allowed ESR can be found by:

(

)

EXRIPPLE

VΔIIESR <+×

Where I

RIPPLE

is the inductor ripple current, ΔI is the maximum

load current step change, and V

is the allowed output voltage

excursion in the transient.

Electrolytic capacitors can be used for the output capacitor, but

are less stable with age than tantalum capacitors. As they age, their

ESR degrades, reducing the system performance and increasing the

risk of failure. It is recommended that multiple parallel capacitors

be used, so that, as ESR increase with age, overall performance

will still meet the processor’s requirements.

There is frequently strong pressure to use the least expensive

components possible; however, this could lead to degraded long-

term reliability, especially in the case of filter capacitors.

Microsemi’s demonstration boards use the CDE Polymer AL-EL

(ESRE) filter capacitors, which are aluminum electrolytic, and

have demonstrated reliability. The OS-CON series from Sanyo

generally provides the very best performance in terms of long term

ESR stability and general reliability, but at a substantial cost

penalty. The CDE Polymer AL-EL (ESRE) filter series provides

excellent ESR performance at a reasonable cost. Beware of off-

brand, very low-cost filter capacitors, which have been shown to

degrade in both ESR and general electrolytic characteristics over

time.

LX1675

PRODUCTION DATA SHEET

Microsemi

Integrated Products Division

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 15

Rev. 1.2a, 2006-02-16

WWW.Microsemi .COM

Multiple Output LoadSHARE™ PWM

APPLICATION NOTE (CONTINUED)

NPUT CAPACITOR

The input capacitor and the input inductor, if used, are to filter

the pulsating current generated by the buck converter to reduce

interference to other circuits connected to the same 5V rail. In

addition, the input capacitor provides local de-coupling for the

buck converter. The capacitor should be rated to handle the RMS

current requirements. The RMS current is:

d)d(1II

LRMS

−=

Where I

is the inductor current and d is the duty cycle. The

maximum value occurs when d = 50% then I

RMS

=0.5I

. For 5V

input and output in the range of 2 to 3V, the required RMS

current is very close to 0.5I

SOFT-START CAPACITOR

The value of the soft-start capacitor determines how fast the

output voltage rises and how large the inductor current is required

to charge the output capacitor. The output voltage will follow the

voltage at the SS pin if the required inductor current does not

exceed the maximum allowable current for the inductor. The SS

pin voltage can be expressed as:

(

)

SSSS

Ct/R

e1VV

−

−= ref

Where R

and C

are the soft-start resistor and capacitor.

The current required to charge the output capacitor during the soft

start interval is.

dVss

CoutIout =

Taking the derivative with respect to time results in

SSSS

Ct/R

RssCss

VrefCout

Iout

−

and at t = 0

RssCss

VrefCout

axIm =

The required inductor current for the output capacitor to follow

the soft start voltage equals the required capacitor current plus the

load current. The soft-start capacitor should be selected to

provide the desired power on sequencing and insure that the

overall inductor current does not exceed its maximum allowable

rating.

Values of C

equal to 0.1µF or greater are unlikely to result in

saturation of the output inductor unless very large output capacitors

are used.

OVER-CURRENT PROTECTION

Current limiting occurs at current level I

when the voltage

detected by the current sense comparator is greater than the current

sense comparator threshold, V

TRIP

(0.0 Volts).

SET SET CL DS(ON) TRIP

I•R-I•R =V

So,

CL DS(ON) CL DS(ON)

SET

IR IR

I50µA

××

Example:

For 10A current limit, using FDS6670A MOSFET (10mΩ

DS(ON)

SET

10 0.010

R2.00KΩ 1%

50 10

−

Note: If R

SET

is 0.0Ω or the CSx pin has become shorted to

ground the device will be continuously in the current limit mode. If

the CSx pin is left open then the current limit will never be enabled.

A resistor should be selected for the maximum desired current limit

and this should also provide enough current to charge up the output

filter capacitance during the soft-start time.

The current limit comparator is followed by a counter that does

not allow the hiccup mode until the current limit condition has

existed for 4 PWM cycles. If the current limit condition goes away

after a count of 2 the counter will be reset. This mode will prevent

a single cycle current or noise glitch from starting the hiccup mode

current limit.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P19

LX1675CLQ-TR

Mfr. #:

Buy LX1675CLQ-TR

Manufacturer:

Microchip / Microsemi

Description:

IC REG QUAD BUCK/LNR SYNC 38MLPQ

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LX1675CLQ-TR

LX1675CLQ-TR

Products related to this Datasheet