MAX1759EUB(2000_01) 데이터 시트보기 (PDF) - Maxim Integrated

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MAX1759EUB
(Rev.:2000_01)

Buck/Boost Regulating Charge Pump in µMAX

Maxim Integrated 

Buck/Boost Regulating

Charge Pump in µMAX

Detailed Description

The MAX1759’s unique charge-pump architecture

allows the input voltage to be higher or lower than the

regulated output voltage. Internal circuitry senses VIN

and VOUT and determines whether VIN must be

stepped up or stepped down to produce the regulated

output. When VIN is lower than VOUT, the charge pump

operates as a regulated step-up voltage doubler. When

VIN is higher than VOUT, the charge pump operates as

a step-down gated switch.

In voltage step-down mode (i.e., the input voltage is

greater than the output voltage) with a light load, the

controller connects CXN to PGND, and shuttles charge

to the output by alternately connecting CXP from IN to

OUT (see Figures 1 and 2). Although VIN is greater than

VOUT, this scheme may not allow the MAX1759 to regu-

late the output under heavy loads. In this case, the

MAX1759 will automatically switch to step-up mode. In

step-up mode, the output is kept in regulation by modu-

lating the charge delivered by the transfer capacitor

(CX) to the load (see Figure 2). When lightly loaded, the

charge pump switches only as necessary to supply the

load, resulting in low quiescent current. Output voltage

ripple does not increase with light loads.

Shutdown Mode

Driving SHDN low places the MAX1759 in shutdown

mode. This disables the charge-pump switches, oscil-

lator, and control logic, reducing quiescent current to

1µA. The output is high impedance in shutdown and is

disconnected from the input. The POK output is high

impedance in shutdown.

Undervoltage Lockout

The MAX1759 undervoltage lockout feature deactivates

the device when the input voltage falls below 1V.

Power-OK Output

POK is an open-drain output that sinks current when

the regulator feedback voltage falls below 1.1V. The

feedback voltage can be either the internal resistor-

divider feedback voltage when in fixed output mode

(FB tied to GND) or an external feedback voltage from

an external resistive divider in adjustable output mode.

A 10kΩ to 1MΩ pull-up resistor from POK to OUT may

be used to provide a logic output. Connect POK to GND

or leave unconnected if not used.

Soft-Start and Short-Circuit Protection

The MAX1759 features foldback short-circuit protec-

tion. This circuitry provides soft-start by limiting inrush

current during startup and limits the output current to

110mA (typ) if the output is short-circuited to ground.

0.33µF

IN

+1.6V TO +5.5V

CXN

IN

CXP

OUT

10µF

MAX1759

ON

OFF

SHDN

POK

FB GND PGND

OUT

3.3V AT 100mA

10µF

100k

POWER OK

Figure 1. Typical Application Circuit

Thermal Shutdown

The MAX1759 features thermal shutdown with tempera-

ture hysteresis. When the die temperature exceeds

160°C, the device shuts down. When the die cools by

20°C, the MAX1759 turns on again. If high die tempera-

ture is caused by output overload and the load is not

removed, the device will turn off and on, resulting in a

pulsed output.

Design Procedure

Setting the Output Voltage

The MAX1759 dual-mode feedback controller selects

between the internally set 3.3V regulated output or an

external resistive divider that allows adjustment of the

output voltage from 2.5V to 5.5V. Connect FB to GND

for a regulated 3.3V output. For an adjustable output,

connect a resistive divider between OUT and GND. To

ensure feedback-loop stability and to minimize error due

to FB pin bias currents, the resistive divider current

should be approximately 15µA. In the following equa-

tion, choose R2 in the 50kΩ to 100kΩ range, and calcu-

late R1 from the following formula (Figure 3):

R1 = R2 [(VOUT / VFB) - 1]

and

VOUT = VFB (R1 + R2) / R2

where VOUT is the desired output voltage from 2.5V to

5.5V, and VFB is the internal regulation voltage, nomi-

nally 1.235V.

The circuit of Figure 3 generates a regulated 2.5V, using

external standard 1% resistor values. Surface-mount

resistors should be placed close to the MAX1759, less

than 5mm away from FB (see the PC Board Layout

section).

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