LTC4253IGN datasheet(20/32 Pages) LINER

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LTC4253

4253f

APPLICATIO S I FOR ATIO

(Refer to Block Diagram)

CB MIN

LMAX

()

(8)

where VCB(MIN) = 40mV represents the guaranteed mini-

mum circuit breaker threshold.

During the initial charging process, the LTC4253 may

operate the MOSFET in current limit, forcing (VACL) be-

tween 80mV to 120mV across RS. The minimum inrush

current is given by:

INRUSH MIN

()=

(9)

Maximum short-circuit current limit is calculated using

the maximum VSENSE. This gives

SHORTCIRCUIT MAX

() =

120

(10)

The TIMER capacitor CT must be selected based on the

slowest expected charging rate; otherwise TIMER might

time out before the load capacitor is fully charged. A value

for CT is calculated based on the maximum time it takes the

load capacitor to charge. That time is given by:

CL CHARGE

SUPPLY MAX

INRUSH MIN

()

•

(11)

The maximum current flowing in the DRAIN pin is given by:

DRN MAX

SUPPLY MAX

DRNCL

()

−

(12)

Approximating a linear charging rate, IDRN drops from

IDRN(MAX) to zero, the IDRN component in Equation (3) can

be approximated with 0.5 • IDRN(MAX). Rearranging the

equation, TIMER capacitor CT is given by:

CL CHARGE

DRN MAX

µ+

()

(

)

•(

•

)

200

(13)

Returning to Equation (3), the TIMER period is calculated

and used in conjunction with VSUPPLY(MAX) and

ISHORTCIRCUIT(MAX) to check the SOA curves of a prospec-

tive MOSFET.

As a numerical design example, consider a 30W load,

which requires 1A input current at 36V. If VSUPPLY(MAX) =

72V and CL = 100µA, RD = 1MΩ, Equation (8) gives

RS = 40mΩ; Equation (13) gives CT = 441nF. To account

for errors in RS, CT, TIMER current (200µA), TIMER

threshold (4V), RD, DRAIN current multiplier and DRAIN

voltage clamp (VDRNCL), the calculated value should be

multiplied by 1.5, giving the nearest standard value of

CT = 680nF.

If a short-circuit occurs, a current of up to

120mV/40m

Ω = 3A will flow in the MOSFET for 3.6ms as

dictated by CT = 680nF in Equation (3). The MOSFET must

be selected based on this criterion. The IRF530S can

handle 100V and 3A for 10ms and is safe to use in this

application.

Computing the maximum soft-start capacitor value during

soft-start to a load short is complicated by the nonlinear

MOSFET’s SOA characteristics and the RSSCSS response.

An overconservative but simple approach begins with the

maximum circuit breaker current, given by:

CB MAX

() =

(14)

From the SOA curves of a prospective MOSFET, determine

the time allowed, tSOA(MAX). CSS is given by:

SOA MAX

()

.•

0 916

(15)

In the above example, 60mV/40m

Ω gives 1.5A. tSOA for

the IRF530S is 40ms. From Equation (15), CSS = 437nF.

Actual board evaluation showed that CSS = 100nF was

appropriate. The ratio ( RSS • CSS ) to tCL(CHARGE) is a good

gauge as large ratios may result in the time-out period

expiring prematurely. This gauge is determined empiri-

cally with board level evaluation.

SUMMARY OF DESIGN FLOW

To summarize the design flow, consider the application

shown in Figure 2. It was designed for 50W and CL = 100µF.

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