LT3154AVPBF datasheet(12/32 Pages) AD

12 / 32 page

LT3154

Rev. 0

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QUICK REFERENCE

Thebuck‑boostconvertercanbesynchronizedtoanexter‑

nal clock applied to the SYNC/MODE pin. The frequency of

theexternalclockmustbehigherthantheinternaloscillator

frequency as set by the RT pin. In order to accommodate

the ±18% possible variation in the oscillator frequency,

the RT resistor should be chosen to set the internal oscil‑

lator frequency between 25% to 50% below the lowest

synchronization frequency. For example, to synchronize

to an external 2.5MHz clock, RT should be selected to set

the internal oscillator at 1.9MHz or lower.

Soft-Start Period

The soft‑start circuit linearly ramps the average inductor

current during the soft‑start period (tSS) from 0A up to

6A. An internal soft‑start interval of approximately 2.2ms

can be selected by connecting SS to VIN. For applications

requiring a longer soft‑start period, an external capacitor

CSS on SS sets soft‑start period according to the formula:

tSS(ms)= 0.8 • CSS(nF)

CSS

PROGRAMMABLE

SOFT-START

DEFAULT 2.2ms

SOFT-START

VIN

3154 QR05

In most applications, the initial load current at VOUT re‑

quires only a fraction of the 6A maximum current limit,

resulting in VOUT achieving regulation in a fraction of the

soft‑start period. Table 2 gives approximate times for VOUT

to regulate for various CSS values and initial load currents

in buck mode.

Table 2. Approximate Time for VOUT to Achieve Regulation vs

Load Current and CSS

Load Current

0.1A

CSS Value

VOUT in Regulation Time (ms)

SS to VIN

0.5

0.75

1.5

10nF

47nF

Inductor Value

While the ideal inductor value is influenced by many fac‑

tors, in most cases, the maximum value is dictated by

the right‑half plane zero (RHPZ) frequency at high load

current in boost mode based on the following equation:

L <

VIN2 •RL

VOUT2 •2π• fRHPZ

Theinternalcontrolloopcrossoverfrequencyvariesbetween

5kHz to 15kHz in boost mode. To get adequate frequency

separation, the inductor value should be chosen to give a

RHPZ frequency > 100kHz. A 0.47µH to 1µH inductor works

well for most applications and load currents with switch‑

ing frequencies above 1MHz. Applications with switching

frequencies below 1MHz may require larger inductor values

and control loop adjustments, at the VC pin, to lower the

crossover frequency to avoid the RHPZ.

Output Capacitor Value

The recommended minimum output capacitor, CMIN, is

given below as a function of output voltage:

CMIN =

VOUT

330µF

Table 3. Minimum Output Capacitor vs VOUT

VOUT

Minimum COUT

68µF

3.3

100µF

1.8

200µF

Control Pin Voltage Limitations

1. SYNC/MODE and EN/UVLO are low voltage input pins

and cannot be forced above 6V.

2. The RT and SS pins should be connected to VIN to

select default values or to GND though the appropriate

external resistor or capacitor when programmed.

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