MAX15021 datasheet(12/24 Pages) MAXIM

12 / 24 page

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

Regulator with Tracking/Sequencing Capability

MAX15021

Maxim Integrated

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 (NCL) 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 (NCL), 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

NCL 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 4.2k

Ω 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 VAVIN

≤ 3V, the switching frequency (fSW) should be derated

to 3MHz (maximum).

[kHz] 1

[V]

4[MHz]

[]

Ω=

067

CURRENT LIMIT

COUNT OF 4

NCL

CLR

INITIATE HICCUP

TIMEOUT

NHT

COUNT OF 3

NCLR

CLR

Figure 3. Hiccup-Mode Block Diagram

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