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LT3597

3597f

APPLICATIONS INFORMATION

Selecting the optimum switching frequency depends

on several factors. Inductor size is reduced with higher

frequency, but efficiency drops slightly due to higher

switching losses. Some applications require very low

duty cycles to drive a small number of LEDs from a high

supply. Low switching frequency allows a greater range

of operational duty cycle and hence a lower number of

LEDs can be driven. In each case, the switching frequency

can be tailored to provide the optimum solution. When

programming the switching frequency, the total power

losses within the IC should be considered.

Switching Frequency Synchronization

The nominal operating frequency of the LT3597 is pro-

grammed using a resistor from the RT pin to ground

over a 200kHz to 1MHz range. In addition, the internal

oscillator can be synchronized to an external clock applied

to the SYNC pin. The synchronizing clock signal input to

the LT3597 must have a frequency between 240kHz and

1MHz, a duty cycle between 20% and 80%, a low state

below 0.4V and a high state above 1.6V. Synchronization

signals outside of these parameters will cause erratic

switching behavior. For proper operation, an RT resistor

is chosen to program a switching frequency 20% slower

than the SYNC pulse frequency. Synchronization occurs

at a fixed delay after the rising edge of SYNC.

The SYNC pin must be grounded if the clock synchroniza-

tion feature is not used. When the SYNC pin is grounded,

the internal oscillator controls the switching frequency of

the converter.

Figure 11. Programming Maximum VOUT1-3

Operating Frequency Trade-offs

Selection of the operating frequency is a trade-off between

efficiency, component size, output voltage and maximum

input voltage. The advantage of high frequency operation

is smaller component sizes and values. The disadvantages

are lower efficiency and lower input voltage range for a

desired output voltage. The highest acceptable switch-

ing frequency (fSW(MAX)) for a given application can be

calculated as follows:

fSW(MAX) =

VD + VOUT

tON(MIN) VD + VIN − VSW

(

)

where VIN is the typical input voltage, VOUT is the output

voltage, VD is the catch diode drop (0.5V) and VSW is the

internal switch drop (0.5V at max load). This equation

showsthatslowerswitchingisnecessarytoaccommodate

high VIN /VOUT ratios. The reason the input voltage range

depends on the switching frequency is due to the finite

minimum switch on and off times. The switch minimum

on and off times are 200ns.

Adaptive Loop Control

The LT3597 uses an adaptive control mechanism to set

the buck output voltage. This control scheme ensures

maximum efficiency while not compromising minimum

PWM pulse widths. When PWM1-3 is low, the output of

the respective buck rises to a maximum value set by an

external resistor divider to the respective FB pin. Once

PWM1-3 goes high, the output voltage is adaptively re-

duced until the voltage across the LED current sink is 1V.

Figure 11 shows how the maximum output voltage can

be set by an external resistor divider.

LT3597

3597 F11

FB1-3

VOUT1-3

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