LTC1629C-PG View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC1629C-PG

PolyPhase, High Efficiency, Synchronous Step-Down Switching Regulators

Linear Technology 

LTC1629/LTC1629-PG

APPLICATIO S I FOR ATIO

RSENSE Selection For Output Current

RSENSE1, 2 are chosen based on the required output

current. The LTC1629 current comparator has a maxi-

mum threshold of 75mV/RSENSE and an input common

mode range of SGND to 1.5( INTVCC). The current com-

parator threshold sets the peak inductor current, yielding

a maximum average output current IMAX equal to the peak

value less half the peak-to-peak ripple current, ∆IL.

Allowing a margin for variations in the LTC1629 and

external component values yields:

RSENSE = (50mV/IMAX)N

where N = number of stages.

When using the controller in very low dropout conditions,

the maximum output current level will be reduced due to

internal slope compensation required to meet stability

criterion for buck regulators operating at greater than 50%

duty factor. A curve is provided to estimate this reduction

in peak output current level depending upon the operating

duty factor.

Operating Frequency

The LTC1629 uses a constant frequency, phase-lockable

architecture with the frequency determined by an internal

capacitor. This capacitor is charged by a fixed current plus

an additional current which is proportional to the voltage

applied to the PLLFLTR pin. Refer to Phase-Locked Loop

and Frequency Synchronization in the Applications Infor-

mation section for additional information.

A graph for the voltage applied to the PLLFLTR pin vs

frequency is given in Figure␣ 2. As the operating frequency

is increased the gate charge losses will be higher, reducing

efficiency (see Efficiency Considerations). The maximum

switching frequency is approximately 310kHz.

Inductor Value Calculation and Output Ripple Current

The operating frequency and inductor selection are inter-

related in that higher operating frequencies allow the use

of smaller inductor and capacitor values. So why would

anyone ever choose to operate at lower frequencies with

larger components? The answer is efficiency. A higher

frequency generally results in lower efficiency because of

2.5

2.0

1.5

1.0

0.5

0

120

170

220

270

320

OPERATING FREQUENCY (kHz)

1629 F02

Figure 2. Operating Frequency vs VPLLFLTR

MOSFET gate charge and transition losses. In addition to

this basic tradeoff, the effect of inductor value on ripple

current and low current operation must also be consid-

ered. The PolyPhase approach reduces both input and

output ripple currents while optimizing individual output

stages to run at a lower fundamental frequency, enhancing

efficiency.

The inductor value has a direct effect on ripple current. The

inductor ripple current ∆IL per individual section, N,

decreases with higher inductance or frequency and in-

creases with higher VIN or VOUT:

∆IL

=

VOUT

fL



1−

VOUT 

VIN 

where f is the individual output stage operating frequency.

In a PolyPhase converter, the net ripple current seen by the

output capacitor is much smaller than the individual

inductor ripple currents due to the ripple cancellation. The

details on how to calculate the net output ripple current

can be found in Application Note 77.

Figure 3 shows the net ripple current seen by the output

capacitors for the different phase configurations. The

output ripple current is plotted for a fixed output voltage as

the duty factor is varied between 10% and 90% on the

x-axis. The output ripple current is normalized against the

inductor ripple current at zero duty factor. The graph can

be used in place of tedious calculations. As shown in

Figure␣ 3, the zero output ripple current is obtained when:

12

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