LT8705_15 datasheet(21/44 Pages) LINER

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LT8705

8705fe

For more information www.linear.com/LT8705

applicaTions inForMaTion

The duty cycle of CLKOUT is proportional to the die tem-

perature and can be used to monitor the die for thermal

issues.SeetheJunctionTemperatureMeasurementsection

for more information.

Inductor Current Sensing and Slope Compensation

TheLT8705operatesusinginductorcurrentmodecontrol.

As described previously in the Power Switch Control sec-

tion,theLT8705measuresthepeakoftheinductorcurrent

waveform in the boost region and the valley of the inductor

current waveform in the buck region. The inductor current

is sensed across the RSENSE resistor with pins CSP and

CSN. During any given cycle, the peak (boost region) or

valley (buck region) of the inductor current is controlled

by the VC pin voltage.

Slope compensation provides stability in constant-

frequency current mode control architectures by prevent-

ing subharmonic oscillations at high duty cycles. This

is accomplished internally by adding a compensating

ramp to the inductor current signal in the boost region,

or subtracting a ramp from the inductor current signal

in the buck region. At higher duty cycles, this results in

a reduction of maximum inductor current in the boost

region, and an increase of the maximum inductor current

in the buck region. For example, refer to the Maximum

Inductor Current Sense Voltage vs Duty Cycle graph in the

Typical Performance Characteristics section. The graph

showsthat,withVCatitsmaximumvoltage,themaximum

inductor sense voltage VRSENSE is between 78mV and

117mV depending on the duty cycle. It also shows that

the maximum inductor valley current in the buck region

is 86mV increasing to ~130mV at higher duty cycles.

RSENSE Selection and Maximum Current

The RSENSEresistancemustbechosenproperlytoachieve

thedesiredamountofoutputcurrent.Toomuchresistance

can limit the output current below the application require-

ments.StartbydeterminingthemaximumallowedRSENSE

resistance in the boost region, RSENSE(MAX,BOOST). Follow

this by finding the maximum allowed RSENSE resistance in

the buck region, RSENSE(MAX,BUCK). The selected RSENSE

resistance must be smaller than both.

Boost Region: In the boost region, the maximum output

current capability is the least when VIN is at its minimum

and VOUT is at its maximum. Therefore RSENSE must be

chosen to meet the output current requirements under

these conditions.

Start by finding the boost region duty cycle when VIN is

minimum and VOUT is maximum using:

DC(MAX,M3,BOOST) ≅ 1–

VIN(MIN)

VOUT(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟•100%

For example, an application with a VIN range of 12V to

48V and VOUT set to 36V will have:

DC(MAX,M3,BOOST) ≅ 1–

12V

36V

⎛

⎝

⎜

⎞

⎠

⎟•100%= 67%

Referring to the Maximum Inductor Current Sense Volt-

age graph in the Typical Performance Characteristics

section, the maximum RSENSE voltage at 67% duty cycle

is ≅93mV, or:

VRSENSE(MAX,BOOST, MAX) ≅93mV

for VIN = 12V, VOUT = 36V.

Next, the inductor ripple current in the boost region must

be determined. If the main inductor L is not known, the

maximum ripple current ∆IL(MAX,BOOST) can be estimated

by choosing ∆IL(MAX,BOOST) to be 30% to 50% of the

maximum inductor current in the boost region as follows:

∆IL(MAX,BOOST) ≅

VOUT(MAX) •IOUT(MAX,BOOST)

VIN(MIN) •

100%

%Ripple

– 0.5

⎛

⎝

⎜

⎞

⎠

⎟

where:

IOUT(MAX,BOOST) is the maximum output load current

required in the boost region

%Ripple is 30% to 50%

For example, using VOUT(MAX) = 36V, VIN(MIN) = 12V,

IOUT(MAX,BOOST)=2Aand%Ripple=40%wecanestimate:

∆IL(MAX,BOOST) ≅

36V •2A

12V • 100%

40%

– 0.5

⎛

⎝

⎜

⎞

⎠

⎟

= 3A

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