MAX1830 Ver la hoja de datos (PDF) - Maxim Integrated

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MAX1830

3A, 1MHz, Low-Voltage, Step-Down Regulators with Synchronous Rectification and Internal Switches

Maxim Integrated 

3A, 1MHz, Low-Voltage, Step-Down Regulators with

Synchronous Rectification and Internal Switches

Table 2. Output Voltage Programming

PIN

OUTPUT VOLTAGE (V)

FBSEL

FB

MAX1830 MAX1831

VCC

Output voltage

2.5

2.5

Unconnected Output voltage

1.5

1.5

REF

Output voltage

1.8

3.3

GND

Resistive divider

Adjustable

LX

MAX1830

MAX1831

VOUT

R2

FB

R1 = 30kΩ

R1

R2 = R1(VOUT / VREF - 1)

VREF = 1.1V

Figure 4. Adjustable Output Voltage

Synchronous Rectification

In a step-down regulator without synchronous rectifica-

tion, an external Schottky diode provides a path for cur-

rent to flow when the inductor is discharging. Replacing

the Schottky diode with a low-resistance NMOS syn-

chronous switch reduces conduction losses and

improves efficiency.

The NMOS synchronous-rectifier switch turns on follow-

ing a short delay after the PMOS power switch turns off,

thus preventing cross conduction or “shoot through.” In

constant-off-time mode, the synchronous-rectifier

switch turns off just prior to the PMOS power switch

turning on. While both switches are off, inductor current

flows through the internal-body diode of the NMOS

switch. The internal-body diode’s forward voltage is rel-

atively high. An external Schottky diode from PGND to

LX can improve efficiency.

Thermal Resistance

Junction-to-ambient thermal resistance, θJA, is highly

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1830/MAX1831

evaluation kit has 0.7in2 of copper area and a thermal

resistance of +71°C/W with no forced airflow. Airflow

over the board significantly reduces the junction-to-

ambient thermal resistance. For heatsinking purposes,

evenly distribute the copper area connected at the IC

among the high-current pins.

Power Dissipation

Power dissipation in the MAX1830/MAX1831 is domi-

nated by conduction losses in the two internal power

switches. Power dissipation due to supply current in the

control section and average current used to charge

and discharge the gate capacitance of the internal

switches (i.e., switching losses) is approximately:

PDS = C x VIN2 x fPWM

where C = 5nF and fPWM is the switching frequen-

cy in PWM mode.

This number is reduced when the switching frequency

decreases as the part enters Idle Mode. Combined con-

duction losses in the two power switches are approxi-

mated by:

PD = IOUT2 x RPMOS

where RPMOS is the on-resistance of the PMOS switch.

The junction-to-ambient thermal resistance required to

dissipate this amount of power is calculated by:

where:

θJA = (TJ,MAX - TA,MAX) / PD(TOT)

θJA = junction-to-ambient thermal resistance

TJ,MAX = maximum junction temperature

TA,MAX = maximum ambient temperature

PD(TOT) = total losses

Design Procedure

For typical applications, use the recommended compo-

nent values in Table 1. For other applications, take the

following steps:

1) Select the desired PWM-mode switching frequency;

1MHz is a good starting point. See Figure 3 for maxi-

mum operating frequency.

2) Select the constant off-time as a function of input

voltage, output voltage, and switching frequency.

3) Select RTOFF as a function of off-time.

4) Select the inductor as a function of output voltage,

off-time, and peak-to-peak inductor current.

Setting the Output Voltage

The output of the MAX1830/MAX1831 is selectable

between one of three preset output voltages. For a pre-

set output voltage, connect FB to the output voltage

and connect FBSEL as indicated in Table 2. For an

adjustable output voltage, connect FBSEL to GND and

connect FB to a resistive divider between the output

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