LT3688 datasheet(13/28 Pages) LINER

13 / 28 page

LT3688

3688f

APPLICATIONS INFORMATION

Setting the Output Voltage

The output voltage is programmed with a resistor divider

between the output and the FB pin. Choose the 1% resistors

according to:

= R2

VOUT

0.8V

–1

⎛

⎝⎜

⎞

⎠⎟

For reference designators, refer to the Block Diagram.

Setting the Switching Frequency

The LT3688 uses a constant-frequency PWM architecture

that can be programmed to switch from 350 kHz to 2.2 MHz

by using a resistor tied from the RT pin to ground. Table 1

shows the RT values for various switching frequencies

Table 1. Switching Frequency vs RT

SWITCHING FREQUENCY (MHz)

(kΩ)

0.35

165

0.5

110

0.6

88.7

0.7

0.8

64.9

0.9

56.2

49.9

1.2

40.2

1.4

33.2

1.6

27.4

1.8

23.2

2.1

2.3

17.4

Operating Frequency Tradeoffs

Selection of the operating frequency is a tradeoff between

efﬁciency, component size and maximum input voltage.

The advantage of high frequency operation is that

smaller inductor and capacitor values may be used. The

disadvantages are lower efﬁciency, and narrower input

voltage range at constant-frequency. The highest constant-

switching frequency (fSW(MAX)) for a given application can

be calculated as follows:

fSW(MAX) =

VOUT + VF

tON(MIN) VIN + VF –VSW

()

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

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

the internal switch drop (~0.3V at maximum load). If the

LT3688 is programmed to operate at a frequency higher

than fSW(MAX) for a given input voltage, the LT3688 enters

pulse skip mode, where it skips switching cycles to maintain

regulation. At frequencies higher than fSW(MAX), the LT3688

no longer operates with constant frequency. The LT3688

enters pulse skip mode at frequencies higher than fSW(MAX)

because of the limitation on the LT3688’s minimum on time

of 140ns (180ns for TJ>125°C).Astheswitchingfrequency

is increased above fSW(MAX), the part is required to switch

for shorter periods to maintain the same duty cycle. Delays

associated with turning off the power switch dictate the

minimum on-time of the part. When the required on-time

decreases below the minimum on-time of 140ns, the switch

pulse width remains ﬁxed at 140ns (instead of becoming

narrower) to accommodate the same duty cycle require-

ment. The inductor current ramps up to a value exceeding

the load current and the output ripple increases. The part

then remains off until the output voltage dips below the

programmed value before it begins switching again.

Maximum Operating Voltage Range

The maximum input voltage for LT3688 applications

depends on switching frequency, the absolute maximum

ratings of the VIN and BST pins, and by the minimum

duty cycle (DCMIN). The LT3688 can operate from input

voltages up to 36V.

DCMIN = tON(MIN) • fSW

where tON(MIN) is equal to 140ns and fSW is the switching

frequency. Running at a lower switching frequency allows

a lower minimum duty cycle. The maximum input voltage

before pulse-skipping occurs depends on the output volt-

age and the minimum duty cycle:

VIN(PS) =

VOUT + VF

DCMIN

–VF + VSW

Example: f = 2.1MHz, VOUT = 3.3V

DCMIN = 140ns • 2.1MHz = 0.294

VIN(PS) =

3.3V

+ 0.5V

0.294

– 0.5V

+ 0.3V = 12.7V

LT3688 Datasheet(PDF) 13 Page - Linear Technology

LT3688 Datasheet(HTML) 13 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL