LT1956IGN datasheet(12/28 Pages) LINER

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LT1956/LT1956-5

1956f

APPLICATIO S I FOR ATIO

load current is required, the inductor value must be

increased. If IOUT(MAX) no longer meets the discontinuous

mode criteria, use the IOUT(MAX) equation for continuous

mode; the LT1956 is designed to operate well in both

modes of operation, allowing a large range of inductor

values to be used.

SHORT-CIRCUIT CONSIDERATIONS

For a ground short-circuit fault on the regulated output,

the maximum input voltage for the LT1956 is typically

limited to 25V. If a greater input voltage is required,

increasing the resistance in series with the inductor may

suffice (see short-circuit calculations at the end of this

section). Alternatively, the 1.5A LT1766 can be used since

it is identical to the LT1956 but runs at a lower frequency

of 200kHz, allowing higher sustained input voltage capa-

bility during output short circuit.

The LT1956 is a current mode controller. It uses the VC

node voltage as an input to a current comparator which

turns off the output switch on a cycle-by-cycle basis as

peak switch current is reached. The internal clamp on the

VC node, nominally 2V, then acts as an output switch peak

current limit. This action becomes the switch current limit

specification. The maximum available output power is

then determined by the switch current limit.

A potential controllability problem could occur under

short-circuit conditions. If the power supply output is

short circuited, the feedback amplifier responds to the low

output voltage by raising the control voltage, VC, to its

peak current limit value. Ideally, the output switch would

be turned on, and then turned off as its current exceeded

the value indicated by VC. However, there is finite response

time involved in both the current comparator and turnoff

of the output switch. These result in a minimum on time

tON(MIN). When combined with the large ratio of VIN to

(VF + I • R), the diode forward voltage plus inductor I • R

voltage drop, the potential exists for a loss of control.

Expressed mathematically the requirement to maintain

control is:

VI R

•

≤

solution. The maximum output load current in discontinu-

ous mode, however, must be calculated and is defined

later in this section.

Discontinuous mode is entered when the output load

current is less than one-half of the inductor ripple current

(ILP-P). In this mode, inductor current falls to zero before

the next switch turn-on (see Figure 8). Buck converters

will be in discontinuous mode for output load current

given by:

Discontinous Mode

Vf L

OUT

()(

–

)

()(

)( )()

The inductor value in a buck converter is usually chosen

large enough to keep inductor ripple current (ILP-P) low;

this is done to minimize output ripple voltage and maxi-

mize output load current. In the case of large inductor

values, as seen in the equation above, discontinuous

mode will be associated with “light loads.”

When choosing small inductor values, however, discon-

tinuous mode will occur at much higher output load

currents. The limit to the smallest inductor value that can

be chosen is set by the LT1956 peak switch current (IP)

and the maximum output load current required given by:

IOUT(MAX)

LP-P

Discontinuous Mode

If L V

OUT

+−

()

()( )(

)

()(

–

)

Example: For VIN = 15V, VOUT = 5V, VF = 0.63V, f = 500kHz

and L = 4

µH

Discontinuous Mode

OUT MAX

()

(

•

)( •

)( )

)(

– – .

)

−

1 5 500 10

4 10

2 5 0 63 1550 63

IOUT(MAX) Discontinuous Mode = 0.639A

What has been shown here is that if high inductor ripple

current and discontinuous mode operation can be toler-

ated, small inductor values can be used. If a higher output

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