MAX1623(1999) Просмотр технического описания (PDF) - Maxim Integrated

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MAX1623
(Rev.:1999)

3A, Low-Voltage, Step-Down Regulator with Synchronous Rectification and Internal Switches

Maxim Integrated 

3A, Low-Voltage, Step-Down Regulator with

Synchronous Rectification and Internal Switches

LX

MAX1623

PGND

GND

FB

R1 = 10kΩ to 500kΩ

R2 = R1(VOUT / VREF - 1)

VREF = 1.1V

VOUT

R2

R1

Figure 3. Adjustable Output Voltage

A high capacitor value maintains a constant average

output voltage but slows the loop response to changes

in output voltage. A low capacitor value speeds up the

loop response to changes in output voltage. Choose

the capacitor value that results in optimal performance.

Current Limiting

The current-sense circuit enables when the PMOS

power switch is on. This circuit’s corresponding output

voltage feeds three separate comparators: the skip cur-

rent comparator (1.25A), the maximum current com-

parator (4.15A), and the PWM current comparator (see

Modes of Operation section).

Oscillator Frequency and

Programming the Off-Time

The MAX1623 features a programmable off-time that is

set by RTOFF connected from TOFF to GND. Connecting

a 110kΩ resistor from TOFF to GND achieves a 1µs

(nominal) off-time. The off-time is inversely proportional

to RTOFF according to the formula:

tOFF = RTOFF / 110k (µs)

tOFF is adjustable between 0.5µs to 4µs (see Typical

Operating Characteristics). To set the switching fre-

quency when the inductor operates in continuous-con-

duction mode, the off-time has to be set to:

tOFF

=

VI − VO − VPCH

f (VI − VPCH + VNCH)

where:

tOFF = the programmed off-time

VI = input voltage

VO = output voltage

f = desired switching frequency during continuous

inductor current

VPCH = the voltage drop across the internal P-channel

switch

VNCH = the voltage drop across the internal N-channel

synchronous rectifier

Switching frequency decreases as load current is

decreased below the 625mA Idle Mode trip point.

Internal Reference

The 1.10V internal reference (available at REF) is accu-

rate to ±1.5% over the -40°C to +85°C operating range,

making it useful as a precision system reference. Bypass

the reference to ground with a minimum 0.1µF ceramic

capacitor. For low noise and jitter performance, use a

0.47µF ceramic capacitor. The reference can supply up

to 10µA for external loads. However, if tight accuracy

specifications for either reference or the main output are

essential, avoid reference loads in excess of 5µA.

Loading the reference reduces the main output voltage

slightly, according to the reference-voltage load-regula-

tion error.

Start-Up

To prevent the MAX1623 from false output regulation,

the internal PMOS and NMOS switches will not switch

on until all of the following conditions are true: the sup-

ply voltage is above the undervoltage lockout thresh-

old, SHDN is pulled high, the internal reference voltage

is at 75% of its nominal (1.1V) value, and the die tem-

perature is below +145°C. When the above conditions

are satisfied, the MAX1623 will regulate the output volt-

age to the selected level. The MAX1623 typically starts

up in 1ms for full output load.

Thermal Shutdown and

Overload Conditions

Thermal overload protection limits the MAX1623’s total

power dissipation. When the junction temperature

reaches Tj = +145°C, the device turns off, allowing it to

cool down. Switching resumes after the IC’s junction

temperature decreases by 20°C. If the thermal overload

condition persists, the output pulses on and off.

Thermal overload protection is designed to protect the

MAX1623 during fault conditions, such as an output

short circuit.

Thermal Resistance

Junction to ambient thermal resistance (θJA) strongly

depends on the amount of copper area immediately

surrounding the IC’s leads. The MAX1623 evaluation kit

has 0.8in2 of copper area. θJA on this board was mea-

sured to have 45°C/W of thermal resistance with no air

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