MAX15034AAUI+T Datasheet MAX15034BAUI+, MAX15034AAUI+, MAX15034BAUI+T | P13-P15

MAX15034

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

______________________________________________________________________________________ 13

Hiccup Fault Protection

The MAX15034 includes overload fault protection circuitry

that prevents damage to the power MOSFETs. The fault

protection consists of two digital fault integration blocks

that enable hiccuping under overcurrent conditions. This

circuit works as follows: for every clock cycle the current-

limit threshold is exceeded, the fault integration counter

increments by one count. Thus, if the current-limit condi-

tion persists, the counter reaches its shutdown threshold

in 32,768 counts and shuts down the external MOSFETs.

When the MAX15034 shuts down due to a fault, the

counter begins to count down (since the current-limit con-

dition has ended), once every 16 clock cycles. Thus, the

device counts down for 524,288 clock cycles. At this

point, switching resumes. This produces an effective duty

cycle of 6.25% power-up and 93.75% power-down under

fault conditions. With a switching frequency set to

250kHz, power-up and power-down times are approxi-

mately 131ms and 2.09s, respectively.

Control Loop

The MAX15034 uses an average current-mode control

topology to regulate the output voltage. The control

loop consists of an inner current loop and an outer volt-

age loop. The inner current loop controls the output

current, while the outer voltage loop controls the output

voltage. The inner current loop absorbs the inductor

pole, reducing the order of the outer voltage loop to

that of a single-pole system. Figure 2 is the block dia-

gram of OUT1’s control loop.

The current loop consists of a current-sense resistor,

SENSE

, a current-sense amplifier (CA1), a current-

error amplifier (CEA1), an oscillator providing the carri-

er ramp, and a PWM comparator (CPWM1). The

precision current-sense amplifier (CA1) amplifies the

sense voltage across R

SENSE

by a factor of 36. The

inverting input to CEA1 senses the output of CA1. The

output of CEA1 is the difference between the voltage-

error amplifier output (EAOUT1) and the gained-up volt-

age from CA1. The RC compensation network

connected to CLP1 provides external frequency com-

pensation for the respective CEA1 (see the

Compensation

section). The start of every clock cycle

enables the high-side driver and initiates a PWM on-

cycle. Comparator CPWM1 compares the output volt-

age from CEA1 against a 0 to 2V ramp from the

oscillator. The PWM on-cycle terminates when the ramp

voltage exceeds the error voltage from the current-error

amplifier (CEA1).

DRIVE

OUT1

OUT

REF

= 0.61V

CFF

CSN1

CSP1

CLP1

P-P

SENSE

LOAD

CA 1

CEA1

CPWM1

VEA1

Figure 2. Current and Voltage Loops

MAX15034

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

14 ______________________________________________________________________________________

The outer voltage control loop consists of the voltage-

error amplifier (VEA1). The noninverting input (EAN1) is

externally connected to the midpoint of a resistive volt-

age-divider from OUT1 to EAN1 to AGND. The voltage

loop gain is set by using an external resistor from the

output of this amplifier (EAOUT1) to its inverting input

(EAN1). The noninverting input of (VEA1) is connected

to the 0.61V internal reference.

Current-Error Amplifier

The MAX15034 features two dedicated transconduc-

tance current-error amplifiers CEA1 and CEA2 with a

typical g

of 550µS and 320µA output sink and source

capability. The current-error amplifier outputs (CLP1 and

CLP2) serve as the inverting input to the PWM compara-

tors. CLP1 and CLP2 are externally accessible to pro-

vide frequency compensation for the inner current loops

(see C

CFF

, C

, and R

in Figure 2). Compensate the

current-error amplifier so that the inductor current down

slope, which becomes the up slope at the inverting

input of the PWM comparator, is less than the slope of

the internally generated voltage ramp (see the

Compensation

section).

PWM Comparator and R-S Flip-Flop

The PWM comparator (CPWM1 or CPWM2) sets the

duty cycle for each cycle by comparing the current-

error amplifier output to a 2V

P-P

ramp. At the start of

each clock cycle an R-S flip-flop resets and the high-

side drivers (DH1 and DH2) turn on. The comparator

sets the flip-flop as soon as the ramp voltage exceeds

the current-error amplifier output voltage, thus terminat-

ing the on-cycle.

Voltage-Error Amplifier

The voltage-error amplifier (VEA_) sets the gain of the

voltage control loop. Its output clamps to 1.14V and

-0.234V relative to V

= 0.61V. Set the MAX15034 out-

put voltage by connecting a voltage-divider from the

output to EAN_ to GND (see Figure 4). At no load, the

output of the voltage error amplifier is zero.

Use the equation below to calculate the no load voltage:

The voltage at full load is given by:

where ∆V

OUT

is the voltage-positioning window

described in the

Adaptive Voltage Positioning

section.

Adaptive Voltage Positioning

Powering new-generation ICs requires new techniques

to reduce cost, size, and power dissipation. Voltage

positioning (Figure 5) reduces the total number of out-

put capacitors to meet a given transient response

requirement. Setting the no-load output voltage slightly

higher than the output voltage during nominally loaded

conditions allows a larger downward voltage excursion

when the output current suddenly increases.

Regulating at a lower output voltage under a heavy

load allows a larger upward-voltage excursion when

the output current suddenly decreases. A larger

allowed voltage-step excursion reduces the required

number of output capacitors and/or allows the use of

higher ESR capacitors.

The MAX15034 internal 0.6125V reference provides a

tolerance of ±1.25%. Using 0.1% resistors for R1 and

R2 allows a 4% variation from the nominal output volt-

age. This available voltage range allows the reduction

of the total number of output capacitors to meet a given

transient response requirement resulting in a voltage-

positioning window as shown in Figure 5.

From the allowable voltage-positioning window calcu-

late the value of R

from the equation below.

where ∆V

OUT

is the allowable voltage-positioning win-

dow, R

SENSE

is the sense resistor, 36 is the current-

sense amplifier gain, and R

is as shown in Figure 4.

IR R

OUT SENSE

OUT

×××36

∆

OUT FL OUT()

.=×+

⎛

⎝

⎜

⎞

⎠

⎟

−0 6125 1

∆

OUT NL()

.=×+

⎛

⎝

⎜

⎞

⎠

⎟

0 6125 1

MAX15034

MOSFET Gate Drivers (DH_, DL_)

The high-side drivers (DH1 and DH2) and low-side dri-

vers (DL1 and DL2) drive the gates of external n-channel

MOSFETs. The high-peak sink and source current capa-

bility of these drivers provides ample drive for the fast

rise and fall times of the switching MOSFETs. Faster rise

and fall times result in reduced switching losses. For low-

output, voltage-regulating applications where the duty

cycle is less than 50%, choose high-side MOSFETs (Q2

and Q4, Figure 6) with a moderate R

DS(ON)

and a very

low gate charge. Choose low-side MOSFETs (Q1 and

Q3, Figure 6) with very low R

DS(ON)

and moderate gate

charge. The driver block also includes a logic circuit that

provides an adaptive nonoverlap time (30ns typ) to pre-

vent shoot-through currents during transition. Figure 7

shows the dual-phase, single-output buck regulator.

2 x f

(V/S)

RAMP

CLK

CSP_

CSN_

EN_

1.225V

CLP_

BST_

DH_

LX_

DL_

PGND

= 36V/V

PWM

COMPARATOR

= 500µS

Figure 3. Current Comparator and MOSFET Driver Logic

LOAD

OUT

REF

= 0.61V

EAN_

EAOUT_

Figure 4. Voltage Error Amplifier

LOAD (A)

CNTR

NO LOAD

1/2 LOAD

FULL LOAD

VOLTAGE-POSITIONING WINDOW

CNTR

+ ∆V

OUT

CNTR

- ∆V

OUT

Figure 5. Defining the Voltage-Positioning Window

Configurable, Single-/Dual-Output, Synchronous

Buck Controller for High-Current Applications

______________________________________________________________________________________ 15

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MAX15034AAUI+T

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Manufacturer:

Maxim Integrated

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Switching Controllers Configurable Synchronous Buck

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MAX15034AAUI+T

MAX15034AAUI+T

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