MAX1742 Просмотр технического описания (PDF) - Maxim Integrated

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MAX1742

1A/2.7A, 1MHz, Step-Down Regulators with Synchronous Rectification and Internal Switches

Maxim Integrated 

1A/2.7A, 1MHz, Step-Down Regulators with

Synchronous Rectification and Internal Switches

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.

Thermal Resistance

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

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1742 evaluation kit

has 0.5in2 of copper area and a thermal resistance of

80°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 MAX1742/MAX1842 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 = 2.5nF 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:

θJA = (TJ,MAX - TA,MAX) / PD(TOT)

where: 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 Tables 1 or 2. 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.

Table 1. MAX1742 Recommended

Component Values (IOUT = 1A)

VIN

VOUT

fPWM

(V)

(V)

(kHz)

L

(μH)

RTOFF

(kΩ)

5

3.3

850

5.6

39

5

2.5

1070

5.6

47

5

1.8

910

5.6

75

5

1.5

770

5.6

100

3.3

2.5

610

3.9

39

3.3

1.8

1050

3.9

43

3.3

1.5

1000

3.9

56

Table 2. MAX1842 Recommended

Component Values (Continuous Output

Current = 1A, Burst Output Current = 2.7A)

VIN

VOUT

fPWM

(V)

(V)

(kHz)

5

3.3

800

5

2.5

1180

5

1.8

850

5

1.5

715

3.3

2.5

570

3.3

1.8

985

3.3

1.5

940

L

(μH)

2.2

2.2

2.2

2.2

1.5

1.5

1.5

RTOFF

(kΩ)

39

47

75

100

39

43

56

MAXIMUM RECOMMENDED

OPERATING FREQUENCY vs. INPUT VOLTAGE

1400

VOUT = 1.5V

1200

1000

800

VOUT = 1.8V

600

VOUT = 2.5V

400

VOUT = 3.3V

200

0

2.6 3.1 3.6 4.1 4.6 5.1 5.6

VIN (V)

Figure 3. Maximum Recommended Operating Frequency vs.

Input Voltage

______________________________________________________________________________________ 11

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