LTM8049 datasheet(13/34 Pages) LINER

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LTM4651

4651f

For more information www.linear.com/LTM4651

The typical LTM4651 application circuit is shown in Test

Circuit 1. External component selection is primarily deter-

mined by the maximum load current and output voltage.

Refer to Table 8 for recommended external component

values.

Output Current Capability Varies as a Function of VIN

to VOUT– Conversion Ratios

The output current capability of the LTM4651 has a strong

dependencyontheoperatinginput(VIN)andoutput(VOUT–)

voltages, as highlighted in the page 1 graph.

The reason for this is inherent in the two-switch buck-

boost topology employed by the LTM4651. To protect

the primary power MOSFET (MT) from overstress (see

Simplified Block Diagram), its peak current (IPK) is limited

by control circuitry to 6A. When MT is on, observe that no

current flows to LTM4651’s output; furthermore, observe

that only when MT is off does current flow to the output

of the LTM4651. As a consequence of this arrangement:

for a given output voltage, current limit inception activates

sooner at low line (higher, larger duty cycle) than at high

line (lower, smaller duty cycle). A further consequence is:

for a given input voltage, the output power capability of

the LTM4651 is higher for lower-magnitude VOUT– (lower,

smaller duty cycle) than for higher-magnitude VOUT–

(higher,largerdutycycle).Thecombinationoftheseeffects

is shown the plots in the page 1 graph and described by

the following equation:

IOUT(CAPABILITY) =

VIN • IPK –

∆IPK–PK











 •η

VIN – VOUT–

(1)

where:

∆IPK-PK is the inductor ripple current, in amps, and η (unit

less) is the efficiency of the LTM4651.

APPLICATIONS INFORMATION

For completeness, ∆IPK-PK is given by:

where:

L is 4μH, the LTM4651’s power inductor value, and fSW is

the switching frequency of the LTM4651, in MHz.

For a practical design, ∆IPK-PK is designed to be less than

~2APK-PK.

For a practical design, the LTM4651’s on-time of MT

each switching cycle should be designed to exceed the

LTM4651 control loop’s specified minimum on-time of

60ns, tON(MIN), (guardband to 90ns) i.e.:

fSW

> TON(MIN)

(3)

where D (unitless) is the duty-cycle of MT, given by:

–VOUT–

VIN – VOUT–

(4)

Combining EQ. 4 with EQ. 1, it can be illustrative to see:

IOUT(CAPABILITY) =(1–D)• IPK –

∆IPK–PK











 •η

(5)

In rare cases where the minimum on-time restriction is

violated, the frequency of the LTM4651 automatically and

gradually folds back down to one-fifth of its programmed

switchingfrequencytoallowVOUT–toremaininregulation.

Be reminded of Notes 2, 3 and 5 in the Electrical Char-

acteristics section regarding output current guidelines.

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