LTC4282 datasheet(13/32 Pages) AD

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LTC4238

Rev. 0

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resistor values available around 1mΩ. Threshold values

as low as 6mV reduce power dissipation in sense resis-

tors for high current applications.

Two current limit foldback profiles are available to meet

different application needs, constant power and high

power. Refer to Table 3 for foldback configurations. The

constant power profile is shaped such that the power in

the MOSFET is constant while current limiting, regardless

of VDS. This simplifies the SOA design of the application

and makes the safe dissipation time a constant for various

voltage and current conditions. This works well with the

current limit timer on the TMR pin. However, the constant

power foldback profile starts to fold back at small VDS,

which could occur during an input step. For that reason

the high power profile is also available. It doesn’t start to

fold back until the VDS is around 50% of the nominal input

voltage. This prevents the current limit from folding back

after an input step and collapsing the output because it is

less than the load current. Since the power dissipation in

the MOSFET is not constant for the high power profile, the

worst-case power dissipation usually occurs when half of

the nominal supply voltage is across the MOSFET. Graphs

in the Typical Performance Characteristics show the cur-

rent limit and power versus voltage across the MOSFETs.

Additionally, to ease start-up, the LTC4238 features a

configurable option for a start-up current at 10% of the

full current limit. The LTC4238 stays at 10% until the

conditions for power good are met, at which point it will

switch to the normal foldback profile and current limit. In

many cases this will eliminate the need for an RC network

on the GATEs of the MOSFETs to limit the inrush current.

Configurations using the 10% current limit for start-up

all use the SOA timer. This timer provides the flexibility

to allow a long current limit timeout at low power levels

during start-up, and a short current limit timeout during

a fault after start-up.

Constant Current Start-Up Using GATE RC Networks

An optional series RC network from GATE to GND (RG

and CG in Figure 5) provides an inrush current less than

the current limit by limiting the slew rate of the GATE

pin. The current limit timer will not run since the current

limit is not engaged during start-up. Thus, a small timer

capacitor may be used which allows the use of MOSFETs

with smaller SOA. Power good will signal when the FB

pin crosses its 2.56V threshold and the ΔVGATE voltages

cross their 8V thresholds. When both those conditions are

met and the impedance back to the supply through the

MOSFET is low, the output voltage is suitable for the load

to be turned on. PG voltage goes high to indicate power

is good. RG should be chosen such that IGATE • RG is less

than the threshold voltage of the MOSFET to avoid an ini-

tial inrush current spike. But increasing RG improves the

stability of the current limit servo loop (see Applications

Information on current limit stability). If the voltage of the

50µA IGATE current across RG is higher than MOSFET's

threshold, a diode may be added in parallel with the large

RG to limit its voltage while charging up CG (see Figure 5).

For the staged-start architectures, an RC network may be

used on a trickle MOSFET or stress MOSFET. In the paral-

lel architecture, identical RC networks may be used on

both MOSFETs. Bypass MOSFETs don’t need the current

Table 1. ISET Pin Voltage* vs Current Limit Thresholds and Suggested 1% Resistor Values

ΔVSNS(TH) (mV)

VISET (V)

ISET Thresholds Compared with

RTOP (kΩ)

RBOTTOM (kΩ)

RBOTTOM/(RTOP + RBOTTOM)

Lower (V)

Upper (V)

0.357

Open

Short

0.000

0.714

0.357

1.071

88.7

14.7

0.143

1.429

1.071

1.786

73.2

29.4

0.286

2.143

1.786

2.5

59.0

44.2

0.429

2.857

2.5

3.214

44.2

59.0

0.571

3.571

3.214

3.929

29.4

73.2

0.714

4.286

3.929

4.643

14.7

88.7

0.857

4.643

Short

Open

1.000

*INTVCC = 5V is used for this table.

APPLICATIONS INFORMATION

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