LTC3251_15 Datasheet, PDF(13/16 Page) Linear Technology – 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

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LTC3251_15 Datasheet, PDF (13/16 Pages) Linear Technology – 500mA High Efficiency, Low Noise, Inductorless Step-Down DC/DC Converter

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OPERATIO (Refer to Block Diagram)

At moderate to high output power the switching losses

and quiescent current of the LTC3251 family is negligible

and the expression above is valid. For example with VIN =

3.6V, IOUT = 200mA and VOUT regulating to 1.5V the

measured efficiency is 81% which is in close agreement

with the theoretical 83.3% calculation.

Programming the LTC3251 Output Voltage (FB Pin)

The LTC3251 is programmed to an arbitrary output volt-

age via an external resistive divider. Figure 7 shows the

required voltage divider connection. The voltage divider

ratio is given by the expression:

RA = VOUT â 1

RB 0.8V

VOUT

LTC3251

GND

CA RA

COUT

( ) VOUT

0.8V

3251 F07

Figure 7. Programming the LTC3251

Typical values for total voltage divider resistance can

range from several kâ¦s up to 1Mâ¦.

The user may want to consider load regulation when

setting the desired output voltage. The closed loop output

impedance of the LTC3251 is approximately:

0.045â¦

â¢

VOUT

0.8V

LTC3251/

LTC3251-1.2/LTC3251-1.5

For a 1.5V output, RO is 0.085â¦, which produces a 40mV

output change for a 500mA load current step. Thus, the

user may want to target an unloaded output voltage

slightly higher than desired to compensate for the output

load conditions. The output may be programmed for

regulation voltages of 0.9V to 1.6V.

Since the LTC3251 employs a 2-to-1 charge pump archi-

tecture, it is not possible to achieve output voltages

greater than half the available input voltage. The minimum

VIN supply required for regulation can be determined by

the following equation:

VIN(MIN) â¥ 2 â¢ (VOUT(MIN) + IOUT â¢ ROL)

The compensation capacitor (CA) is necessary to counter-

act the pole caused by the large valued resistors RA and RB,

and the input capacitance of the FB pin. For best results, CA

should be 5pF for all RA or RB greater than 10k and can be

omitted if both RA and RB are less than 10k.

Disabling Spread Spectrum Operation on the

LTC3251-1.2/LTC3251-1.5 (MODE Pin)

Spread spectrum operation can be disabled by driving

MODE high. When Mode is high, switching takes place at

the maximum operating frequency (typ 1.6MHz). The

advantage of spread spectrum operation is that it reduces

the peak noise at and above the operating frequency at the

expense of a slightly increased noise floor and slightly

increased low frequency ripple caused by the converter

compensating for the changing operating frequency. Us-

ers who do not need the peak noise reduction gained by

using spread spectrum may wish to disable spread spec-

trum, thus improving the low frequency input/output

ripple.

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