LTC1735 datasheet(10/24 Pages) LINER

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LTC1778/LTC1778-1

1778fb

APPLICATIO S I FOR ATIO

The basic LTC1778 application circuit is shown in

Figure 1. External component selection is primarily de-

termined by the maximum load current and begins with

the selection of the sense resistance and power MOSFET

switches. The LTC1778 uses the on-resistance of the

synchronous power MOSFET for determining the induc-

tor current. The desired amount of ripple current and

operating frequency largely determines the inductor value.

Finally, CIN is selected for its ability to handle the large

RMS current into the converter and COUT is chosen with

low enough ESR to meet the output voltage ripple and

transient specification.

Choosing the LTC1778 or LTC1778-1

The LTC1778 has an open-drain PGOOD output that

indicates when the output voltage is within ±7.5% of the

regulation point. The LTC1778-1 trades the PGOOD pin for

a VON pin that allows the on-time to be adjusted. Tying the

VON pin high results in lower values for RON which is useful

in high VOUT applications. The VON pin also provides a

means to adjust the on-time to maintain constant fre-

quency operation in applications where VOUT changes and

to correct minor frequency shifts with changes in load

current. Finally, the VON pin can be used to provide

additional current limiting in positive-to-negative convert-

ers and as a control input to synchronize the switching

frequency with a phase locked loop.

Maximum Sense Voltage and VRNG Pin

Inductor current is determined by measuring the voltage

across a sense resistance that appears between the PGND

and SW pins. The maximum sense voltage is set by the

voltage applied to the VRNG pin and is equal to approxi-

mately (0.133)VRNG. The current mode control loop will

not allow the inductor current valleys to exceed

(0.133)VRNG/RSENSE. In practice, one should allow some

margin for variations in the LTC1778 and external compo-

nent values and a good guide for selecting the sense

resistance is:

SENSE

RNG

OUT MAX

10 •

()

An external resistive divider from INTVCC can be used to

set the voltage of the VRNG pin between 0.5V and 2V

resulting in nominal sense voltages of 50mV to 200mV.

Additionally, the VRNG pin can be tied to SGND or INTVCC

in which case the nominal sense voltage defaults to 70mV

or 140mV, respectively. The maximum allowed sense

voltage is about 1.33 times this nominal value.

Power MOSFET Selection

The LTC1778 requires two external N-channel power

MOSFETs, one for the top (main) switch and one for the

bottom (synchronous) switch. Important parameters for

the power MOSFETs are the breakdown voltage V(BR)DSS,

threshold voltage V(GS)TH, on-resistance RDS(ON), reverse

transfer capacitance CRSS and maximum current IDS(MAX).

The gate drive voltage is set by the 5V INTVCC supply.

Consequently, logic-level threshold MOSFETs must be

used in LTC1778 applications. If the input voltage is

expected to drop below 5V, then sub-logic level threshold

MOSFETs should be considered.

When the bottom MOSFET is used as the current sense

element, particular attention must be paid to its on-

resistance. MOSFET on-resistance is typically specified

with a maximum value RDS(ON)(MAX) at 25°C. In this case,

additional margin is required to accommodate the rise in

MOSFET on-resistance with temperature:

DS ON MAX

SENSE

()(

) =

The ρT term is a normalization factor (unity at 25°C)

accounting for the significant variation in on-resistance

Figure 2. RDS(ON) vs. Temperature

JUNCTION TEMPERATURE (°C)

–50

1.0

1.5

150

1778 F02

0.5

100

2.0

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