MAX15014 datasheet(16/25 Pages) MAXIM

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capacitor physically close to the MAX15014–MAX15017

to provide good bypassing. During normal operation, REG

is intended for powering up only the internal circuitry and

should not be used to supply power to external loads.

Soft-Start and Reference (SS)

SS is the 1.235V reference bypass connection for the

MAX15014–MAX15017 and also controls the soft-start

period. At startup, after input voltage is applied at IN_SW,

IN_LDO and the UVLO thresholds are reached, the

device enters soft-start. During soft-start, 14μA is sourced

into the capacitor (CSS) connected from SS to SGND

causing the reference voltage to ramp up slowly. When

VSS reaches 1.244V, the output becomes fully active. Set

the soft-start time (tSS) using following equation:

where VSS = soft-start reference voltage = 1.235V

(typ), ISS = soft-start current = 14 x 10-6A (typ), tSS is in

seconds and CSS is in Farads.

Internal Charge Pump (MAX15015/MAX15016)

The MAX15015/MAX15016 feature an internal charge

pump to enhance the turn-on of the internal MOSFET,

allowing for operation with input voltages down to 4.5V.

Connect a flying capacitor (CF) between C+ and C-,

a boost diode from C+ to BST, as well as a bootstrap

capacitor (CBST) between BST and LX to provide the

gate-drive voltage for the high-side n-channel DMOS

switch. During the on-time, the flying capacitor is charged

to VDVREG. During the off-time, the positive terminal of

the flying capacitor (C+) is pumped to two times VDVREG,

and charge is dumped onto CBST to provide twice the

regulator voltage across the high-side DMOS driver. Use

a ceramic capacitor of at least 0.1μF for CBST and CF,

located as close as possible to the device.

Gate-Drive Supply (DVREG)

DVREG is the supply input for the internal high-side

MOSFET driver. The power for DVREG is derived from

the output of the internal regulator (REG). Connect

DVREG to REG externally. To filter the switching noise,

the use of an RC filter (1Ω and 0.47μF) from REG to

DVREG is recommended. In the MAX15015/MAX15016,

the high-side drive supply is generated using the inter-

nal charge pump along with the bootstrap diode and

capacitor. In the MAX15014/MAX15017, the high-side

MOSFET driver supply is generated using only the boot-

strap diode and capacitor.

Error Amplifier

The output of the internal error amplifier (COMP) is avail-

able for frequency compensation (see the Compensation

Design section). The inverting input is FB, the noninvert-

ing input SS, and the output COMP. The error amplifier

has an 80dB open-loop gain and a 1.8MHz GBW product.

See the Typical Operating Characteristics for the Gain

and Phase vs. Frequency graph.

Oscillator/Synchronization Input (SYNC)

With SYNC connected to SGND, the MAX15014–

MAX15017 use their internal oscillator and switch at a

fixed frequency of 135kHz and 500kHz. The MAX15014/

MAX15016 are the 135kHz options and MAX15015/

MAX15017 are the 500kHz options. For external synchro-

nization, drive SYNC with an external clock from 400kHz

to 600kHz (MAX15015/MAX15017), or 100kHz to 200kHz

(MAX15014/MAX15016). When driven with an external

clock, the device synchronizes to the rising edge of SYNC.

PWM Comparator/Voltage Feed-Forward

An internal ramp generator clocked by the internal

oscillator is compared against the output of the error

amplifier to generate the PWM signal. The maximum

amplitude of the ramp (VRAMP) automatically adjusts to

compensate for input voltage and oscillator frequency

changes. This causes the VIN_SW/VRAMP to be a con-

stant 10V/V across the input voltage range of 4.5V to 40V

(MAX15015/MAX15016), or 7.5V to 40V (MAX15014/

MAX15017), and the SYNC frequency range of 400kHz

to 600kHz (MAX15015/MAX15017), or 100kHz to 200kHz

(MAX15014/MAX15016).

Output Short-Circuit Protection

(Hiccup Mode)

The MAX15014–MAX15017 protect against an output

short circuit by utilizing hiccup-mode protection. In hiccup

mode, a series of sequential cycle-by-cycle current-limit

events cause the part to shut down and restart with a soft-

start sequence. This allows the device to operate with a

continuous output short circuit.

During normal operation, the current is monitored at the

drain of the internal power MOSFET. When the current

limit is exceeded, the internal power MOSFET turns off until

the next on-cycle and a counter increments. If the counter

counts seven consecutive current-limit events, the device

discharges the soft-start capacitor and shuts down for 512

clock periods before restarting with a soft-start sequence.

Each time the power MOSFET turns on and the device

does not exceed the current limit, the counter is reset.

MAX15014–MAX15017

1A, 4.5V to 40V Input Buck Converters with

50mA Auxiliary LDO Regulators

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