LTC3775 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3775

High Frequency Synchronous Step-Down Voltage Mode DC/DC Controller

Linear Technology 

LTC3775

APPLICATIONS INFORMATION

The value of QG should come from the plot of VGS vs

QG in the Typical Performance Characteristics section of

the MOSFET data sheet. The value listed in the electrical

specifications may be measured at a higher VGS, such

as 10V, whereas the value of interest is at the 5V INTVCC

gate drive voltage.

Care must be taken to ensure that the maximum junction

temperature of the LTC3775 is never exceeded. The junc-

tion temperature can be estimated using the following

equations:

PDISS = VIN • IINTVCC

TJ = TA + PDISS • RTH(JA)

As an example of the required thermal analysis, consider

a buck converter with a 24V input voltage and an output

voltage of 3.3V at 15A. The switching frequency is 500kHz

and the maximum ambient temperature is 70°C. The power

MOSFET used for this application is the Vishay Siliconix

Si7884DP, which has a typical RDS(ON) of 7.5mΩ at VGS

= 4.5V and 5.5mΩ at VGS = 10V. From the plot of VGS vs

QG, the total gate charge at VGS = 5V is 18.5nC (the tem-

perature coefficient of the gate charge is low). One power

MOSFET is used for the top side and one for the bottom

side. For the UD package:

IINTVCC = 3.5mA + 2 • 18.5nC • 500kHz = 22mA

PDISS = 24V • 22mA = 528mW

TJ = 70°C + 528mW • 68°C/W = 105.9°C

In this example, the junction temperature rise is 35.9°C.

These equations demonstrate how the gate charge cur-

rent typically dominates the quiescent current of the IC,

and how the choice of the operating frequency and board

heat sinking can have a significant effect on the thermal

performance of the solution.

To prevent the maximum junction temperature from be-

ing exceeded, the input supply current of the IC should

be checked when operating in continuous mode (heavy

load) at maximum VIN. A trade-off between the operat-

ing frequency and the size of the power MOSFETs may

need to be made in order to maintain a reliable junction

temperature.

Finally, it is important to verify the calculations by perform-

ing a thermal analysis of the final PCB using an infrared

camera or thermal probe.

Operation at Low Supply Voltage

The LTC3775 has a minimum input voltage of 4.5V. The

gate driver for the LTC3775 consists of a PMOS pull-up

and an NMOS pull-down device, allowing the full INTVCC

voltage to be applied to the gates during power MOSFET

switching. Nonetheless, care should be taken to deter-

mine the minimum gate drive supply voltage (INTVCC) in

order to choose the optimum power MOSFETs. Important

parameters that can affect the minimum gate drive volt-

age are the minimum input voltage (VIN(MIN)), the LDO

dropout voltage, the QG of the power MOSFETs, and the

operating frequency.

If the input voltage VIN is low enough for the INTVCC LDO

to be in dropout, then the minimum gate drive supply

voltage is:

VINTVCC = VIN(MIN) – VDROPOUT

The LDO dropout voltage is a function of the total gate

drive current and the quiescent current of the IC (typically

3.5mA). A curve of dropout voltage versus output cur-

rent for the LDO is shown in Figure 10. The temperature

coefficient of the LDO dropout voltage is approximately

6000ppm/°C. See the INTVCC Regulator and Thermal

Considerations sections for information about calculating

the total quiescent current.

0

TA = 25°C

–0.2

–0.4

–0.6

–0.8

–1.0

0

10

20

30

40

50

INTVCC LOAD CURRENT (mA)

3775 F10

Figure 10. INTVCC LDO Dropout Voltage vs Current

3775fa

17

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