MAX15034AAUI+ Datasheet MAX15034BAUI+, MAX15034AAUI+, MAX15034BAUI+T | P22-P24

MAX15034

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

22 ______________________________________________________________________________________

The MAX15034 provides precision average current-limit

programmability while using standard sense resistors

or shunts. Use the equation below to determine the

appropriate V

AVGLIMIT

external reference voltage at

AVGLIMIT:

For example, assuming the desired average current

limit is 18A, and R

SENSE

= 2mΩ.

where R

SENSE

is determined from maximum load cur-

rent, wattage rating, and circuit parasitics (see above)

and I

LOAD(MAX)

from circuit requirements. V

AVGLIMIT

the average current-limit reference voltage selected for

a desired I

LOAD(MAX)

and is set by a resistive voltage-

divider from REG to AGND. See the

Programming the

Average Current Limit

section.

The second 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:

Programming the Average Current Limit

The MAX15034 average current-limit reference voltage

is set by connecting a resistor-divider network from

REG to AGND, the center node is connected to

AVGLIMIT. The resistive divider’s upper resistor, R1, is

connected between REG and the AVGLIMIT. The resis-

tive divider’s lower resistor, R2, is connected between

the AVGLIMIT and AGND.

The resistor-divider values are determined by first,

choosing R2. To minimize reference noise select R2

such that (R1 + R2) < 100kΩ; a typical value is 10kΩ.

Next, determine R1 from:

From the example above, assuming V

AVGLIMIT

= 1.91V:

A standard value for R1 is 16.2kΩ. Connect AVGLIMIT

to AGND for default current limit

20 4.

SENSE

⎛

⎝

⎜

⎞

⎠

⎟

k110

191

11618=× −

⎛

⎝

⎜

⎞

⎠

⎟

=ΩΩ

REG

AVGLIMIT

AVGLIMT

12 1

=× −

⎛

⎝

⎜

⎞

⎠

⎟

=×Ω

(()

[]

MAX

−

⎛

⎝

⎜

⎞

⎠

⎟

AVG SHORT LOAD MAX() ()

.=×0 0625

VmAmV

mV V

AVGLIMIT

=××

()

2 36 18 612 5

1910 1 91

Ω .

VRmIA

AVGLIMIT SENSE LOAD MAX

=× ×

()

+56 [ ] [ ]

()

Ω 6612 5.m

ESL

SENSE RESISTOR (INDUCTIVE)

SENSE

OUT

LX_

CSP_

MAX15034

CSN_

OUT

Figure 10. Inductive Sense Resistor

MAX15034

Reverse Current Limit

The MAX15034 limits the reverse current when the out-

put capacitor voltage is higher than the preset output

voltage. Calculate the maximum reverse current limit

based on V

CLMP_LO

and the current-sense resistor

SENSE

Output-Voltage Setting

The output voltage is set by the combination of resistors

R1, R2, and R

as described in the

Voltage-Error

Amplifier

section. First select a value for resistor R2. Then

calculate the value of R1 from the following equation:

where V

OUT(NL)

is the voltage at no load. Then find the

value of R

from the following equation:

where ∆V

OUT

is the allowable drop in voltage from no

load to full load. R

is R8 and R9, R1 is R4 and R6, R2

is R5 and R7 in Figure 6.

Compensation

The MAX15034 uses an average current-mode control

scheme to regulate the output voltage (see Figure 2).

The main control loop consists of an inner current loop

and an outer voltage loop. The voltage error amplifier

(VEA1 and VEA2) provides the controlling voltage for

the current loop in each phase. The output inductor is

hidden inside the inner current loop. This simplifies the

design of the outer voltage control loop and also

improves the power-supply dynamics. The objective of

the inner current loop is to control the average inductor

current. The gain-bandwidth characteristic of the cur-

rent loop can be tailored for optimum performance by

the compensation network at the output of the current-

error amplifier (CEA1 or CEA2). Compared with peak

current-mode control, the current-loop gain crossover

frequency, f

, can be made approximately the same,

but the gain at low frequencies is much higher. This

results in the following advantages over peak current-

mode control.

1) The average current tracks the programmed cur-

rent with a high degree of accuracy.

2) Slope compensation is not required, but there is a

limit to the loop gain at the switching frequency to

achieve stability.

3) Noise immunity is excellent.

4) The average current-mode method can be used to

sense and control the current in any circuit branch.

For stability of the current loop, the amplified inductor-

current downslope at the negative input of the PWM

comparator (CPWM1 and CPWM2) must not exceed

the ramp slope at the comparator’s positive input. This

puts an upper limit on the current-error amplifier gain at

the switching frequency. The inductor current downs-

lope is given by V

OUT

/L where L is the value of the

inductor (L1 and L2 in Figure 6) and V

OUT

is the output

voltage. The amplified inductor current downslope at

the negative input of the PWM comparator is given by:

where R

SENSE

is the current-sense resistor (R1 and R2

in Figure 6) and g

x R

is the gain of the current-error

amplifier (CEA_) at the switching frequency. The slope

of the ramp at the positive input of the PWM comparator

is 2V x f

. Use the following equation to calculate the

maximum value of R

(R14 or R15 in Figure 6).

The highest crossover frequency f

CMAX

is given by:

or alternatively:

Equation (1) can now be rewritten as:

VR g

IN S m

××

×××

2()

CMAX OUT

××2π

CMAX

SW IN

OUT

×2π

VR g

OUT SENSE m

≤

××

×××

1()

∆

RgR

OUT

SENSE m C

=× ×××36

IR R

OUT SENSE

OUT

×××36

∆

OUT NL

0 6125

(.)

()

−

REVERSE

SENSE

−

155 10

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

______________________________________________________________________________________ 23

MAX15034

In practical applications, pick the crossover frequency

) in the range of:

First calculate R

in equation 2 above. Calculate C

so that:

where C

is C10 and C12 in Figure 6.

Calculate C

CFF

so that:

where C

CFF

is C11 and C13 in Figure 6.

Applications Information

Independent Turn-On and Turn-Off

The MAX15034 can be used to regulate two outputs

from one controller. Each of the two outputs can be

turned on and off independently of one another by con-

trolling the enable input of each phase (EN1 and EN2).

A logic-low on each enable pin shuts down the

MOSFET drivers for that phase. When the voltage on the

enable pin exceeds 1.2V, the drivers are turned on and

the output can come up to regulation. This method of

turning on the outputs allows the MAX15034 to be used

for power sequencing.

PCB Layout Guidelines

Careful PCB layout is critical to achieve low losses, low

output noise, and clean and stable operation. This is

especially true for dual-phase converters where one

channel can affect the other. Use the following guide-

lines for PCB layout:

1) Place the V

, REG, and the BST1 and BST2 bypass

capacitors close to the MAX15034.

2) Minimize all high-current switching loops.

3) Keep the power traces and load connections short.

This practice is essential for high efficiency. Use

thick copper PCBs (2oz or higher) to enhance effi-

ciency and minimize trace inductance and resis-

tance.

4) Run the current-sense lines CSP_ and CSN_ very

close to each other to minimize loop areas. Do not

cross these critical signal lines through power cir-

cuitry. Sense the current right at the pads of the

current-sense resistors.

5) Place the bank of output capacitors close to the

load.

6) Isolate the power components on the top side from

the analog components on the bottom side with a

ground plane in between.

7) Provide enough copper area around the switching

MOSFETs, inductors, and sense resistors to aid in

thermal dissipation and reducing resistance.

8) Distribute the power components evenly across the

top side for proper heat dissipation.

9) Keep AGND and PGND isolated and connect them

at one single point close to the IC. Do not connect

them together anywhere else.

10) Place all input bypass capacitors for each input as

close to each other as is practical.

CFF

CCF

×× × ×

210π

CCF

×× ×

2 π

10 2

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

24 ______________________________________________________________________________________

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MAX15034AAUI+

Mfr. #:

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Switching Controllers Configurable Synchronous Buck

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MAX15034AAUI+

MAX15034AAUI+

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