MAX16992ATCD datasheet(13/21 Pages) MAXIM

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n-Channel MOSFET Driver

DRV drives the gate of an external n-channel MOSFET.

The driver is powered by the internal regulator (PVL),

which provides approximately 5V. This makes both the

devices suitable for use with logic-level MOSFETs. DRV

can source 750mA and sink 1000mA peak current.

The average current sourced by DRV depends on the

switching frequency and total gate charge of the external

MOSFET (see the Power Dissipation section).

Error Amplifier

The devices include an internal transconductance error

amplifier. The noninverting input of the error amplifier is

connected to the internal 1V reference and feedback is

provided at the inverting input. High 700FS open-loop

transconductance and 50M

Ω output impedance allow

good closed-loop bandwidth and transient response.

Moreover, the source and sink current capability of

140FA provides fast error correction during output load

transients.

Slope Compensation

The devices use an internal current-ramp generator for

slope compensation. The internal ramp signal resets at

the beginning of each cycle and slews at a typical rate of

50FA x fSW. The amount of slope compensation needed

depends on the slope of the current ramp in the inductor.

See the Current-Sense Resistor Selection and Setting

Slope Compensation section for further information.

Current Limit

The current-sense resistor (RCS) connected between the

source of the MOSFET and ground sets the current limit.

The ISNS input has a voltage trip level (VCS) of 250mV.

When the voltage produced by the current in the induc-

tor exceeds the current-limit comparator threshold, the

MOSFET driver (DRV) quickly terminates the on-cycle.

In some cases, a short time-constant RC filter could be

required to filter out the leading-edge spike on the sense

waveform in addition to the internal blanking time. The

amplitude and width of the leading edge spike depends

on the gate capacitance, drain capacitance, and switch-

ing speed (MOSFET turn-on time).

Hiccup Operation

The devices incorporate a hiccup mode in an effort to

protect the external power components when there is

an output short-circuit. If PGOOD is low (i.e., the output

voltage is less than 85% of its set value) and there are

16 consecutive current-limit events, switching is stopped.

There is then a waiting period of 44ms before the

device tries to restart by initiating a soft-start. Note that

a short-circuit on the output places considerable stress

on all the power components even with hiccup mode,

so that careful component selection is important if this

condition is encountered. For more complete protection

against output short-circuits, a series pMOS switch driven

from PGOOD through a level-shifter can be employed

(see Figure 1).

Applications Information

Inductor Selection

Using the following equation, calculate the minimum

inductor value so that the converter remains in continu-

ous mode operation at minimum output current (IOMIN):

LMIN = (VIN2 x D x E)/(2 x fSW x VOUT x IOMIN)

where:

D = (VOUT + VD - VIN)/(VOUT + VD - VDS)

and:

IOMIN is between 10% and 25% of IOUT

A higher value of IOMIN reduces the required inductance;

however, it increases the peak and RMS currents in the

switching MOSFET and inductor. Select IOMIN between

10% to 25% of the full load current. VD is the forward

voltage drop of the external Schottky diode, D is the duty

cycle, and VDS is the voltage drop across the external

switch. Select an inductor with low DC resistance and

with a saturation current (ISAT) rating higher than the

peak switch current limit of the converter.

Input and Output Capacitors

The input current to a boost converter is almost

continuous and the RMS ripple current at the input

capacitor is low. Calculate the minimum input capacitor

value and maximum ESR using the following equations:

CIN = DIL x D/(4 x fSW x DVQ)

ESRMAX = DVESR/DIL

where DIL = ((VIN - VDS) x D)/(L x fSW).

VDS is the total voltage drop across the external

MOSFET plus the voltage drop across the inductor

ESR. DIL is peak-to-peak inductor ripple current as

calculated above. DVQ is the portion of input ripple due

to the capacitor discharge and DVESR is the contribution

due to ESR of the capacitor. Assume the input capacitor

ripple contribution due to ESR (DVESR) and capacitor

discharge (DVQ) are equal when using a combination of

ceramic and aluminium capacitors. During the converter

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MAX16990/MAX16992

36V, 2.5MHz Automotive Boost/

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