MAX15021ATI+ Datasheet MAX15021ATI+T, MAX15021ATI+, MAX15021EVKIT+ | P10-P12

MAX15021

Dual, 4A/2A, 4MHz, Step-Down

DC-DC Regulator withTracking/

Sequencing Capability

Maxim Integrated | 10www.maximintegrated.com

Detailed Description

The MAX15021 incorporates dual-output, PWM, step-

down, DC-DC regulators with tracking and sequencing

options. The device operates over the input-voltage

range of 2.5V to 5.5V. Each PWM regulator provides an

adjustable output down to 0.6V and delivers up to 4A

(regulator 1) and 2A (regulator 2) of load current. The

high switching frequency (up to 4MHz) and integrated

power switches optimize the MAX15021 for high-perfor-

mance and small-size power management solutions.

Each of the MAX15021 PWM regulator sections utilizes

a voltage-mode control scheme for good noise immuni-

ty and offers external compensation allowing for maxi-

mum flexibility with a wide selection of inductor values

and capacitor types. The device operates at a fixed

switching frequency that is programmable from 500kHz

to 4MHz with a single resistor. Operating the regulators

with 180° out-of-phase clocking, and at frequencies up

to 4MHz, significantly reduces the RMS input ripple

current. The resulting peak input current reduction (and

increase in the ripple frequency) significantly reduces

the required amount of input bypass capacitance.

The MAX15021 provides coincident tracking, ratiomet-

ric tracking, or sequencing to allow tailoring of power-

up/power-down sequence depending on the system

requirements. When sequencing, it powers up glitch-

free into a prebiased output.

The MAX15021 includes internal undervoltage lockout

with hysteresis, digital soft-start/soft-stop for “glitch-free”

power-up and power-down. Protection features include

lossless, cycle-by-cycle current limit, hiccup-mode out-

put short-circuit protection, and thermal shutdown.

Undervoltage Lockout (UVLO)

The supply voltage (V

AVIN

) must exceed the default

UVLO threshold before any operation starts. The UVLO

circuitry keeps the MOSFET drivers, oscillator, and all

the internal circuitry shut down to reduce current con-

sumption. The UVLO rising threshold is 2.2V (typ) with

a 120mV (typ) hysteresis.

Digital Soft-Start/Soft-Stop

The MAX15021 soft-start feature allows the load volt-

age to ramp up in a controlled manner, eliminating out-

put-voltage overshoot. Soft-start begins after V

AVIN

exceeds the undervoltage lockout threshold and the

enable input is above 1.225V (typ). The soft-start cir-

cuitry ramps up the reference voltage, controlling the

rate of rise of the output voltage, and reducing input

surge currents during startup. The soft-start duration is

4096 clock cycles. The output voltage is incremented

through 64 equal steps. The output reaches regulation

when soft-start is completed, regardless of the output

capacitance and load.

For tracking applications, soft-stop commences when the

enable input falls below 1.1V (typ). The soft-stop circuitry

ramps down the reference voltage controlling the output-

voltage rate of fall. The output voltage is decremented

through 64 equal steps in 4096 clock cycles.

Oscillator

Use an external resistor at RT to program the

MAX15021 switching frequency from 500kHz to 4MHz.

Calculate the appropriate resistor at RT for the desired

output switching frequency (f

Tracking/Sequencing

The MAX15021 features coincident/ratiometric tracking

and sequencing (see Figure 1). Connect SEL to ground

to configure the device as sequencer. Connect SEL to

AVIN for tracking with output 1 as the master. Leave SEL

unconnected for tracking with output 2 as the master.

Assign the output with the higher voltage as the master.

[]

Ω=

f [kHz] 1 [V]

4[MHz]

067

32 μ

OUT1

SOFT-START

SOFT-STOP

a) COINCIDENT TRACKING OUTPUTS

b) RATIOMETRIC TRACKING OUTPUTS

c) SEQUENCED OUTPUTS

SOFT-STOP

OUT2

OUT1

OUT2

OUT1

OUT2

Figure 1. Graphical Representation of Coincident Tracking,

Ratiometric Tracking, and Sequencing

MAX15021

Dual, 4A/2A, 4MHz, Step-Down

DC-DC Regulator withTracking/

Sequencing Capability

Maxim Integrated | 11www.maximintegrated.com

Coincident/Ratiometric Tracking

The enable inputs in conjunction with digital soft-start

and soft-stop provide coincident/ratiometric tracking.

Track an output voltage by connecting a resistive

divider from the output being tracked to its enable

input. For example, for V

OUT2

to coincidentally track

OUT1

, connect the same resistive divider used for

FB2, from V

OUT1

to EN2 to SGND (see Figure 2).

Track ratiometrically by connecting EN_ to SGND. This

synchonizes the soft-start and soft-stop of all the regu-

lator references, and hence their respective output volt-

ages will track ratiometrically (see Figure 2).

When the MAX15021 regulators are configured as volt-

age trackers, output short-circuit fault conditions at

either master or slave output are handled carefully—nei-

ther the master nor slave output will remain energized

when the other output is shorted to ground. When the

slave is shorted and enters hiccup mode, the master will

soft-stop. When the master is shorted and the part

enters in hiccup mode, the slave will ratiometrically soft-

stop. Coming out of hiccup mode, both outputs will soft-

start coincidently or ratiometrically depending on their

initial configuration. During the thermal shutdown or

power-off when the input falls below its UVLO, the out-

put voltages decrease at a rate depending on the

respective output capacitance and load.

See Figure 1 for a graphical representation of coinci-

dent/ratiometric tracking.

Sequencing

When sequencing, the voltage at the enable inputs

must exceed 1.225V (typ) for each PWM controller to

start (see Figure 1c).

RIN1

EN2

EN1

SEL AVIN

OUTPUT 1 IS THE MASTER AND

OUTPUT 2 IS THE SLAVE.

RATIOMETRIC TRACKING

COINCIDENT TRACKING

SEL

AVIN

OUTPUT 1 IS THE MASTER AND

OUTPUT 2 IS THE SLAVE.

RIN1

EN1

OUT1

EN2

OUT2

FB2

SEL

UNCONNECTED

OUTPUT 2 IS THE MASTER AND

OUTPUT 1 IS THE SLAVE.

RIN2

EN2

OUT2

EN1

OUT1

FB1

RIN2

EN1

EN2

SEL UNCONNECTED

OUTPUT 2 IS THE MASTER AND

OUTPUT 1 IS THE SLAVE.

Figure 2. Ratiometric Tracking and Coincident Tracking Configurations

MAX15021

Dual, 4A/2A, 4MHz, Step-Down

DC-DC Regulator withTracking/

Sequencing Capability

Maxim Integrated | 12www.maximintegrated.com

Error Amplifier

The output of the internal voltage-mode error amplifier

(COMP_) is provided for frequency compensation (see

the

Compensation-Design Guidelines

section). FB_ is

the inverting input of the error amplifier. The error

amplifier has an 80dB open-loop gain and a 12MHz

gain bandwidth (GBW) product.

Output Short-Circuit

Protection (Hiccup Mode)

The MAX15021 features lossless, high-side peak cur-

rent limit and low-side, valley current limit. At short duty

cycles, both limits are active. At high duty cycles, only

the high-side peak current limit is active. Either limit

causes the hiccup mode count (N

) to increment.

For duty cycles less than 50%, the low-side valley cur-

rent limit is active. Once the high-side MOSFET turns off,

the voltage across the low-side MOSFET is monitored. If

this voltage does not exceed the current-limit threshold

at the end of the cycle, the high-side MOSFET turns on

normally at the start of the next cycle. If the voltage

exceeds the current-limit threshold just before the

beginning of a new PWM cycle, the controller skips that

cycle. During severe overload or short-circuit condi-

tions, the switching frequency of the device appears to

decrease because the on-time of the low-side MOSFET

extends beyond a clock cycle.

If the current-limit threshold is exceeded for more than

four cumulative clock cycles (N

), the device shuts

down for 8192 clock cycles (hiccup timeout) and then

restarts with a soft-start sequence. If three consecutive

cycles pass without a current-limit event, the count of

is cleared (see Figure 3). Hiccup mode protects

the device against a continuous output short circuit.

The internal current limit is constant from 5.5V down to

3V and decreases linearly by 50% from 3V to 2V. See

the

Electrical Characteristics

table.

Thermal-Overload Protection

The MAX15021 features an integrated thermal-overload

protection with temperature hysteresis. Thermal-over-

load protection limits the total power dissipation in the

device and protects it in the event of an extended ther-

mal fault condition. When the die temperature exceeds

+160°C, an internal thermal sensor shuts down the

device, turning off the internal power MOSFETs and

allowing the die to cool. After the die temperature falls

by +15°C, the part restarts with a soft-start sequence.

Startup into a Prebiased Output

(Sequencing Mode)

In sequencing mode, the regulators start into a prebi-

ased output and soft-stop is disabled. During soft-start,

the complementary switching sequence is inhibited until

the PWM comparator commands its first PWM pulse.

Until then, the converters do not sink current from the

outputs. The first PWM pulse occurs when the ramping

reference voltage increases above the FB_ voltage.

PWM Controllers

Design Procedure

Setting the Switching Frequency

Connect a 4.2kΩ to 33kΩ resistor from RT to SGND to

program the switching frequency from 500kHz to

4MHz. Calculate the resistor connected to RT using the

following equation:

Higher frequencies allow designs with lower inductor

values and less output capacitance. At higher switch-

ing frequencies core losses, gate-charge currents, and

switching losses increase. When operating from V

AVIN

≤ 3V, the switching frequency (f

) should be derated

to 3MHz (maximum).

f [kHz] 1 [V]

32 4[MHz]

[]

Ω=

067

CURRENT LIMIT

COUNT OF 4

CLR

INITIATE HICCUP

TIMEOUT

COUNT OF 3

CLR

Figure 3. Hiccup-Mode Block Diagram

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MAX15021ATI+

Mfr. #:

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Manufacturer:

Maxim Integrated

Description:

Switching Voltage Regulators Dual 4A/2A 4MHz w/Tracking/Seq

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