LTC3778 查看數據表（PDF） - Linear Technology

零件编号

产品描述 (功能)

比赛名单

LTC3778

Wide Operating Range, No RSENSE™ Step-Down Controller

Linear Technology 

LTC3778

APPLICATIO S I FOR ATIO

The basic LTC3778 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 LTC3778 can use either a sense resistor or

the on-resistance of the synchronous power MOSFET for

determining the inductor current. The desired amount of

ripple current and operating frequency largely deter-

mines 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.

Maximum Sense Voltage and VRNG Pin

Inductor current is determined by measuring the voltage

across a sense resistance that appears between the SENSE–

and SENSE+ pins. The maximum sense voltage is set by

the voltage applied to the VRNG pin and is equal to

approximately (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 LTC3778 and external compo-

nent values and a good guide for selecting the sense

resistance is:

RSENSE

=

10

VRNG

• IOUT(MAX)

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.

Connecting the SENSE+ and SENSE– Pins

The LTC3778 can be used with or without a sense resistor.

When using a sense resistor, it is placed between the

source of the bottom MOSFET, M2, and PGND. Connect

the SENSE+ and SENSE– pins to the top and bottom of the

sense resistor. Using a sense resistor provides a well

defined current limit, but adds cost and reduces efficiency.

Alternatively, one can eliminate the sense resistor and use

the bottom MOSFET as the current sense element by

simply connecting the SENSE+ pin to the SW pin and

SENSE– pin to PGND. This improves efficiency, but one

must carefully choose the MOSFET on-resistance as dis-

cussed below.

Power MOSFET Selection

The LTC3778 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 and DRVCC

supplies. Consequently, logic-level threshold MOSFETs

must be used in LTC3778 applications. If the input voltage

or DRVCC 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:

RDS(ON)(MAX)

=

RSENSE

ρT

3778f

10

Share Link: