LTC3370 datasheet(33/44 Pages) LINER

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LTC3372

Rev. A

For more information www.analog.com

With the input voltage at a sustained 60V and full load on

the output, the estimated power losses in the main FET

will be 2.0W and the LDO losses inside the chip will be

0.8W. Airflow will likely be required. A lower switching

frequency will reduce these losses and setting VOUT to

5V will avoid the LDO losses.

A short-circuit to ground will result in a folded back cur-

rent of:

ISC =

0.45(75mV)

0.005

−

60ns(22V)

2.2µH

⎛

⎝

⎜

⎞

⎠

⎟ = 6.6A

with a maximum value of RDS(ON) and = (0.005/°C)(25°C)

= 0.125. The resulting power dissipated in the bottom

MOSFET is:

PSYNC = (6.6A)

2(1.125)(0.009Ω)

= 441mW

which is less than under full-load conditions. CINischosen

for an RMS current rating of at least 5A at temperature

assuming only this channel is on. COUT is chosen with an

ESR of 0.01Ω for low output ripple. The output ripple in

continuousmodewillbehighestatthemaximuminputvolt-

age.TheoutputvoltagerippleduetoESRisapproximately:

VORIPPLE = RESR(∆IL) = 0.1Ω(4.3A) = 43mVP-P

LOW VOLTAGE BUCK REGULATORS

Output Voltage and Feedback Network

The output voltage of each LV buck switching regulators

is programmed by a resistor divider connected from the

switching regulator’s output to its feedback pin and is

given by VOUT = VFB(1 + R2/R1) as shown in Figure 8.

Typical values for R1 range from 40k to 1M. The buck

APPLICATIONS INFORMATION

regulatortransientresponsemayimprovewithanoptional

capacitor, CFF, that helps cancel the pole created by the

feedback resistors and the input capacitance of the FB pin.

Experimentation with capacitor values between 2.2pF and

22pF may improve transient response.

Input and Output Decoupling Capacitor Selection

The LTC3372 buck regulators have individual input supply

pins for each buck power stage. Each of these pins must

be decoupled with low ESR capacitors to GND. These ca-

pacitors must be placed as close to the pins as possible.

Ceramic dielectric capacitors are a good compromise

between high dielectric constant and stability versus

temperature and DC bias. Note that the capacitance of a

capacitor deteriorates at higher DC bias. It is important

to consult manufacturer data sheets and obtain the true

capacitance of a capacitor at the DC bias voltage that it

will be operated at. For this reason, avoid the use of Y5V

dielectric capacitors. The X5R/X7R dielectric capacitors

offer good overall performance.

Each low voltage input power supply voltage VIN,A-H Pins

5, 8, 9, 12, 25, 28, 29 and 32 all need to be de-coupled

withatleast10μFcapacitors.Ifpowerstagesarecombined

the supplies should be shorted with as short of a trace

as possible. Additionally, all LV buck regulator outputs

should be bypassed with a capacitor to ground of at least

22µF for 1A, 47µF for 2A, 68µF for 3A and 100µF for 4A

configurations.

Combined Buck Power Stages

The LTC3372 has eight power stages that can handle aver-

age load currents of 1A each. These power stages may be

combined in any one of eight possible combinations, via

the C1, C2, and C3 pins (see Table 1). Table 3, Table 4, and

Table 5 show recommended inductors for the combined

power stage configurations.

For a 2A combined buck regulator, the input supply should

bede-coupledwitha22μFcapacitorwhiletheoutputshould

be de-coupled with a 47μF capacitor. Similarly, for 3A and

4A configurations the input and output capacitance must

be scaled up to account for the increased load.

LV BUCK

SWITCHING

REGULATOR

VOUT/EXTVCC

FBn

3372 F08

CFF

OPTIONAL

COUT

Figure 8. Feedback Components

Low Voltage Buck Regulators

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