LTM8049 datasheet(9/20 Pages) LINER

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LTM4661

4661f

For more information www.linear.com/LTM4661

TIME (µs)

3.5

3.0

2.0

1.0

2.5

1.5

0.5

1.0

4661 F01

1.5

0.5

SINGLE PHASE

DUAL

PHASE

Figure 1. Comparison of Output Ripple Current with

Single Phase and Dual Phase Boost Converter

Input Decoupling Capacitors

The LTM4661 module should be connected to a low AC-

impedance DC source. For each module, one piece 10µF

input ceramic capacitor is required for RMS ripple current

decoupling. Bulk input capacitor is only needed when the

inputsourceimpedanceiscompromisedbylonginductive

leads, traces or not enough source capacitance. The bulk

capacitor can be an electrolytic aluminum capacitor and

polymer capacitor.

Output Decoupling Capacitors

With an optimized high frequency, high bandwidth, two

phase interleaving design, only single piece of 22µF low

ESRoutputceramiccapacitorisrequiredforeachLTM4661

module to achieve low output voltage ripple and very

good transient response. Additional output filtering may

be required by the system designer, if further reduction of

output ripples or dynamic transient spikes is required. The

LTpowerCAD® Design Tool is available to download online

for output ripple, stability and transient response analysis.

Soft-Start

The LTM4661 contains internal circuitry to provide soft-

start operation. The soft-start utilizes a linearly increasing

ramp of the error amplifier reference voltage from zero

to its nominal value of 1.2V in approximately 10ms, with

the internal control loop driving VOUT from zero to its final

programmed value. This limits the inrush current drawn

APPLICATIONS INFORMATION

from the input source. As a result, the duration of the

soft-start is largely unaffected by the size of the output

capacitor or the output regulation voltage. The soft-start

period is reset by a shutdown command on SDB, a UVLO

event on INTVCC (INTVCC < 1.5V), an overvoltage event on

VOUT (VOUT ≥ 16.5V), or an overtemperature event (TSD is

invoked when the die temperature exceeds 170°C). Upon

removal of these fault conditions, the LTM4661 will soft-

start the output voltage.

Burst Mode Operation

In applications where high efficiency at light load current

are more important than output voltage ripple, Burst Mode

operation could be used by connecting SYNC/MODE pin to

GND to improve light load efficiency. The output current

(IOUT) capability in Burst Mode operation is significantly

less than in continuous current mode (CCM) and varies

with VIN and VOUT, as shown in Figure 2. The LTM4661

will operate in CCM mode even if Burst Mode operation

is commanded during soft-start.

In Burst Mode operation, only one phase of the LTM4661

isoperational,whiletheotherphaseisdisabled.Thephase

inductor current is initially charged to approximately

700mA by turning on the N-channel MOSFET switch,

at which point the N-channel switch is turned off and

the P-channel synchronous switch is turned on, deliv-

ering current to the output. When the inductor current

discharges to approximately zero, the cycle repeats. In

VIN, FALLING (V)

300

400

4661 F02

200

100

250

350

150

1.5

0.5

2.5

3.5

4.5

5.5

VOUT = 2.5V

VOUT = 5V

VOUT = 7.5V

VOUT = 12V

VOUT = 15V

Figure 2. Burst Mode Output Current vs VIN

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