MAX8561 Просмотр технического описания (PDF) - Maxim Integrated

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MAX8561

4MHz, 500mA Synchronous Step-Down DC-DC Converters in Thin SOT and TDFN

Maxim Integrated 

4MHz, 500mA Synchronous Step-Down

DC-DC Converters in Thin SOT and TDFN

Shutdown Mode

Connecting SHDN to GND or logic low places the

MAX8560/MAX8561/MAX8562 in shutdown mode and

reduces supply current to 0.1µA. In shutdown, the con-

trol circuitry, internal-switching P-channel MOSFET, and

synchronous rectifier (N-channel MOSFET) turn off and

LX becomes high impedance. Connect SHDN to IN or

logic high for normal operation.

Soft-Start

The MAX8560/MAX8561/MAX8562 have internal soft-

start circuitry that eliminates inrush current at startup,

reducing transients on the input source. Soft-start is par-

ticularly useful for higher impedance input sources,

such as Li+ and alkaline cells. See the Soft-Start and

Shutdown Response graphs in the Typical Operating

Characteristics section.

Open-Drain Output

The 8-pin TDFN versions, the MAX8561 and MAX8562,

include an extra, internal, open-drain N-channel MOSFET

switch that can save an additional package in space-con-

strained applications. The open drain is connected to

ODO, while the gate is controlled by a digital input at

ODI. For the MAX8561, this circuit can be used to toggle

between two regulated output voltages, as in Figure 2.

For the MAX8562, a 10kΩ resistor pulls ODO up to IN

when ODI is low, and the buck converter is forced into

100% duty cycle when ODI is high. This makes the

MAX8562 ideal for driving an external bypass PFET for

high-power mode in CDMA cell phones, as in Figure 3.

Applications Information

The MAX8560/MAX8561/MAX8562 are optimized for

use with tiny inductors and small ceramic capacitors.

The correct selection of external components, especial-

ly CFF, ensures high efficiency, low output ripple, and

fast transient response.

Setting the Output Voltage

Select an output voltage between 0.6V and 2.5V by

connecting FB to a resistive voltage-divider between LX

and GND (see the Typical Operating Circuit). Choose

R2 for a reasonable bias current in the resistive divider.

A wide range of resistor values is acceptable, but a

good starting point is to choose R2 as 100kΩ. Then, R1

is given by:

R1 =

R2





VOUT

VFB

−



1

where VFB = 0.6V.

SHDN

PWM

LOGIC

MAX8560

MAX8561*

MAX8562*

ODI

IN

PFET

NFET

LX

0.6V

10kΩ

PGND

(GND)**

FB

ODO

GND

*NOTE: ODI/ODO AVAILABLE IN THE MAX8561/MAX8562 ONLY.

THE MAX8561 ODO IS AN OPEN-DRAIN OUTPUT. THE MAX8562

HAS AN INTERNAL 10kΩ PULLUP TO IN.

**GND FOR MAX8560.

Figure 1. Simplified Functional Diagram

Inductor Selection

The MAX8560/MAX8561/MAX8562 operate with inductors

of 1µH to 4.7µH. Low inductance values are smaller but

require faster switching, which results in some efficiency

loss. See the Typical Operating Characteristics section

for efficiency and switching frequency vs. inductor value.

The inductor’s DC current rating only needs to match the

maximum load current of the application + 50mA

because the MAX8560/MAX8561/ MAX8562 feature zero

current overshoot during startup and load transients.

For output voltages above 2.0V, when light-load efficien-

cy is important, the minimum recommended inductor is

2.2µH. For optimum voltage-positioning load transients,

choose an inductor with DC series resistance in the

50mΩ to 150mΩ range. For higher efficiency at heavy

loads (above 200mA) or minimal load regulation (but

some transient overshoot), the resistance should be kept

below 100mΩ. For light-load applications up to 200mA,

much higher resistance is acceptable with very little

impact on performance.

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