LTC3370 datasheet(32/44 Pages) LINER

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LTC3372

Rev. A

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A second, more severe transient is caused by switching

in loads with large (>1μF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in paral-

lel with COUT, causing a rapid drop in VOUT/EXTVCC. No

regulator can alter its delivery of current quickly enough

to prevent this sudden step change in output voltage if

the load switch resistance is low and it is driven quickly. If

the ratio of CLOAD to COUT is greater than 1:50, the switch

rise time should be controlled so that the load rise time is

limited to approximately 25 • CLOAD. Thus a 10μF capaci-

tor would require a 250μs rise time, limiting the charging

current to about 200mA.

Design Example

As a design example, assume VIN = 12V (nominal),

VIN = 60V (max) for transient voltages, VOUT = 3.3V,

IMAX = 10A, VSENSE(MAX) = 75mV and f = 333kHz. The

inductance value is chosen first based on a 30% ripple

current assumption. The highest value of ripple current

occurs at the maximum input voltage. Tie the RT pin to

INTVCC (for better frequency accuracy over temperature,

use an external RT = 400k to ground), generating 333kHz

operation for LTC3372’s HV controller (1/6 of fOSC at

2MHz). The maximum ripple current is:

ΔIL =

VOUT

(f)(L)

1−

VOUT

VIN(MAX)

⎛

⎝

⎜⎜

⎞

⎠

⎟⎟

A 2.2μH inductor will produce 43% ripple current which

is sufficiently close. The peak inductor current will be the

maximum DC value plus one half the ripple current, or

12.1A. Increasing the ripple current will also help ensure

that the minimum on-time of 60ns is not violated. The

minimum on-time occurs at maximum VIN:

tON(MIN) =

VOUT

VIN(MAX)(f)

3.3V

60V(333kHz)

= 165ns

The equivalent RSENSE resistor value can be calculated by

using the minimum value for the maximum current sense

threshold (68mV):

RSENSE ≤

68mV

12.1A

≈ 5.6mΩ

Rounding down to the next standard value yields a sense

resistor value of 5mΩ.

For a 3.3V/10A 333kHz converter operating with a nominal

12V input and a 60V surge rating, a reasonable MOSFET

selection is:

Top: RJK0651DPB

RDS(ON) = 0.018Ω, QG = 15nC,

CMILLER = 148pF, VTHMIN = 1.2V, VDSS = 60V

Bottom: RJK0652DPB

RDS(ON) = 0.009Ω, QG = 29nC, VDSS = 60V

Atthenominalinputvoltageof12VwithT(estimated) = 50°C,

the losses become:

PMAIN =

3.3V

12V

(10A)2 [

+ (12V)

2 10 A

(2Ω)(148pF) •

5.1V − 1.2V

1.2V

⎡

⎣

⎢

⎤

⎦

⎥(333kHz)

= 643mW

557mW + 77mW

PSYNC =

12V − 3.3V

12V

(10A)2

1+ (0.005)(50°C − 25°C)

[

](0.009Ω)

= 734mW

•

[1 + (0.005)(50°C – 25°C)](0.018)

Other losses include the gate drive and INTVCC LDO

losses. These can be estimated from the gate charge and

switching frequency:

IGQ = 333kHz (15nC + 29nC)

= 14.7mA

PLDO = (12V – 5.1V) 14.7mA

= 101mW

PGATE_DRIVE = 5.1V • 14.7mA

PGATE_DRIVE = 75mW

APPLICATIONS INFORMATION High Voltage Buck Controller

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