MAX1644 データシートの表示（PDF） - Maxim Integrated

部品番号

コンポーネント説明

メーカー

MAX1644

2A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches

Maxim Integrated 

2A, Low-Voltage, Step-Down Regulator with

Synchronous Rectification and Internal Switches

FBSEL

0.01μF

SS

FB

COMP

470pF

REF

10Ω

VIN

VCC

2.2μF

SHDN

FEEDBACK

SELECTION

Gm

REF

MAX1644

SUMMING

COMPARATOR

CURRENT

SENSE

SKIP

PWM LOGIC

AND

DRIVERS

IN

10μF

VIN

3.0V TO 5.5V

VOUT

LX

COUT

REF

1μF

REF

GND

NOTE: HEAVY LINES DENOTE HIGH-CURRENT PATHS.

TIMER

CURRENT

SENSE

TOFF

RTOFF

PGND

Figure 1. Functional Diagram

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.

Thermal Resistance

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

dependent on the amount of copper area immediately

surrounding the IC leads. The MAX1644 evaluation kit

has 0.5 in.2 of copper area and a thermal resistance of

60°C/W with no airflow. Airflow over the IC 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 MAX1644 is dominated 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 dis-

charge the gate capacitance of the internal switches

are less than 30mW at 300kHz. This number is reduced

when the switching frequency decreases as the part

enters Idle Mode. Combined conduction losses in the

two power switches are approximated by:

PD = IOUT2 · RON

The junction-to-ambient thermal resistance required to

dissipate this amount of power is calculated by:

θJA = (TJ,MAX - TA,MAX) / PD

where: θJA = junction-to-ambient thermal resistance

TJ,MAX = maximum junction temperature

TA,MAX = maximum ambient temperature

_______________________________________________________________________________________ 7

Share Link: