LTC4252-2 datasheet(21/34 Pages) LINER

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LTC4253/LTC4253A

425353afe

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) =

CB(MAX)

(14)

where VCB(MAX) is 60mV (55mV for the LTC4253A).

From the SOA curves of a prospective MOSFET, determine

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

SS =

SOA(MAX)

0.916 • R

for the LTC4253

SS =

SOA(MAX)

2.48 • R

for the LTC4253A

(15)

In the above example, 60mV/40mΩ gives 1.5A. tSOA for

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

Actual board evaluation showed that CSS = 100nF was ap-

propriate. 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 empirically

with board level evaluation.

SUMMARY OF DESIGN FLOW

To summarize the design flow, consider the application

shown in Figure 3 for the LTC4253A. It was designed for

80W and CL = 100μF.

Calculate maximum load current: 80W/43V = 1.86A;

allowing for 83% converter efficiency, IIN(MAX) = 2.2A.

Calculate RS: from Equation (8) RS = 20mΩ.

Calculate ISHORT-CIRCUIT(MAX): from Equation (10)

ISHORTCIRCUIT(MAX) = 3.3A.

Select a MOSFET that can handle 3.3A at 71V: IRF530S.

Calculate CT: from Equation (13) CT = 302nF. Select

CT = 680nF, which gives the circuit breaker time-out

period tMAX = 5.9ms.

Consult MOSFET SOA curves: the IRF530S can handle 3.3A

at 100V for 8.3ms, so it is safe to use in this application.

Calculate CSS: using Equations (14) and (15) select

CSS = 33nF.

FREQUENCY COMPENSATION

The LTC4253 typical frequency compensation network

for the analog current limit loop is a series RC (10Ω)

and CC connected from GATE to VEE. Figure 6 depicts the

relationship between the compensation capacitor CC and

the MOSFET’s CISS. The line in Figure 6 is used to select

a starting value for CC based upon the MOSFET’s CISS

specification. Optimized values for CC are shown for sev-

eral popular MOSFETs. Differences in the optimized value

of CC versus the starting value are small. Nevertheless,

compensation values should be verified by board level

short-circuit testing.

As seen in Figure 5, at the onset of a short-circuit event,

the input supply voltage can ring dramatically due to series

inductance. If this voltage avalanches the MOSFET, current

continues to flow through the MOSFET to the output. The

analog current limit loop cannot control this current flow

and therefore the loop undershoots. This effect cannot be

eliminated by frequency compensation. A Zener diode is

required to clamp the input supply voltage and prevent

MOSFET avalanche.

Figure 6. Recommended Compensation

Capacitor CC vs MOSFET CISS for the LTC4253

MOSFET CISS (pF)

4253 F06

2000

4000

6000

8000

IRF530

IRF540

IRF740

IRF3710

NTY100N10

APPLICATIONS INFORMATION

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