Rainbow-electronics MAX5066 Bedienungsanleitung PDF herunterladen (Seite 19)

MAX5066

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

______________________________________________________________________________________ 19

load step. Use a combination of SP polymer and

ceramic capacitors for better transient load and rip-

ple/noise performance.

Keep the maximum output-voltage deviation less than

or equal to the adaptive voltage-positioning window

(∆V

OUT

). During a load step, assume a 50% contribu-

tion each from the output capacitance discharge and

the voltage drop across the ESR (∆V

OUT

= ∆V

ESR_

OUT

+ ∆V

Q_OUT

). Use the following equations to calculate

the required ESR and capacitance value:

where I

LOAD_STEP

is the step in load current and

RESPONSE

is the response time of the controller.

Controller response time depends on the control-loop

bandwidth. C

OUT

is C6 and C7 in Figure 6.

Current Limit

The average current-mode control technique of the

MAX5066 accurately limits the maximum average out-

put current per phase. The MAX5066 senses the volt-

age across the sense resistor and limits the maximum

inductor current accordingly. Use the equations below

to calculate the current-sense resistor values:

Due to tolerances involved, the minimum average volt-

age at which the voltage across the current-sense

resistor is clamped is 20.4mV. Therefore, the minimum

average current limit is set at:

For example, the current-sense resistor:

for a maximum output current of 10A. The standard

value is 2mΩ. Also, adjust the value of the current-

sense resistor to compensate for parasitics associated

with the PC board. Select a noninductive resistor with

appropriate wattage rating.

The second type of current limit is the peak current limit

as explained in the Peak-Current Comparator section.

The third current-protection circuit is the hiccup fault

protection as explained in the Hiccup Fault Protection

section. The average current during a short at the out-

put is given by:

Reverse Current Limit

The MAX5066 limits the reverse current when the output

capacitor voltage is higher than the preset output volt-

age. Calculate the maximum reverse current limit based

on V

CLAMP_LO

and the current-sense resistor R

SENSE

AVG SHORT

SENSE

()

−

141 10

SENSE

20 4

204

. Ω

LIMIT MIN

SENSE

()

−

20 4 10

LOAD MAX

SENSE

()

−

24 75 10

ESR OUT

LOAD STEP

OUT

LOAD STEP RESPONSE

Q OUT

∆

LOSS DESCRIPTION SEGMENT LOSSES

Conduction Loss

Losses associated with MOSFET on-time, I

RMS

is a function of load current and duty cycle.

Gate Drive Loss

Losses associated with charging and

discharging the gate of the MOSFET every

cycle. There is no Q

charging involved in this

MOSFET due to the zero-voltage turn-on. The

charge involved is (Q

- Q

Table 2. Low-Side MOSFET Losses

PIR

where I

CONDUCTION RMS DS ON

RMS

IN OUT

LOAD

=×

≈

−

()

PVQQf

GATEDRIVE DD G GD SW

=×− ×()

Note: The gate drive losses are distributed between the drivers and the MOSFETs in the ratio of the gate driver’s resistance and the

MOSFET’s internal gate resistance.

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