MAX1765(2000) View Datasheet(PDF) - Maxim Integrated

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Description

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MAX1765
(Rev.:2000)

800mA, Low-Noise, Step-Up DC-DC Converter with 500mA Linear Regulator

Maxim Integrated 

800mA, Low-Noise, Step-Up DC-DC Converter

with 500mA Linear Regulator

Pin Description (continued)

PIN

NAME

FUNCTION

9

ONB

ON Input. When low, the DC-DC is operational (Table 2).

10

PGND

Power Ground

11

LX

Inductor connection to the drain of P-channel synchronous rectifier and N-channel switch.

12

POUT

Boost Converter Power Output. POUT is the source of the P-channel synchronous-rectifier MOSFET

switch. Connect POUT to INL. Bypass POUT to PGND with a 100µF capacitor.

13

INL

Linear Regulator Power Input. Source of PFET pass device connected between INL and OUTL.

Connect INL to POUT.

14

OUTL

Linear Regulator Output. OUTL can source up to 500mA. Bypass OUTL to GND with a 4.7µF

capacitor.

Track-Mode Control Input for DC-DC Converter. In track mode, the boost converter output is sensed

15

TRACK

at OUT and set to 0.5V above OUTL to improve efficiency. Set TRACK to OUT for track mode and to

GND for normal operation (Table 2).

16

ONL

Linear Regulator ON Input. Enables the linear regulator output when TRACK = LOW. ONA and ONB

determine the linear regulator’s output state when TRACK = HIGH.

Detailed Description

The MAX1765 is a highly efficient, low-noise power

supply for portable RF hand-held instruments. This

boost power supply combines an LDO linear regulator,

a low-noise, high-power, step-up switching regulator,

an N-channel power MOSFET, a P-channel synchro-

nous rectifier, shutdown control, and a precision volt-

age reference in a single 16-pin QSOP or a thermally

enhanced TSSOP-EP (Figure 1).

The switching DC-DC converter boosts a 1-cell to 3-cell

NiMH/NiCd or a single Li+ battery input to an

adjustable output voltage between 2.5V and 5.5V. The

MAX1765 guarantees startup with voltages as low as

1.1V and will remain operational down to 0.7V (Figure

2). The internal LDO regulator provides linear postregu-

lation for noise-sensitive circuitry, or it can be used as a

separate voltage output adjustable from 1.25V up to

POUT.

The MAX1765 is optimized for use in cellular phones

and other applications requiring low noise during full-

power operation, as well as low quiescent current for

maximum battery life during standby and shutdown.

The device automatically transitions to a low-quiescent-

current pulse-skipping control scheme during light

loads that reduces the quiescent power consumption to

360µW. The supply current of the device can be further

reduced to 1µA when the device is shut down. Figure 2

shows a typical application of the MAX1765 in normal

mode.

The switching regulator supports two low-noise modes:

fixed-frequency PWM for low noise in all load condi-

tions, and synchronization of the internal oscillator to an

external clock driving the CLK input. In TRACK mode,

the DC and linear regulator work together to maintain

excellent PSRR without excessive efficiency loss.

Additional MAX1765 features include synchronous rec-

tification for high efficiency and increased battery life,

dual boost shutdown controls for µP or a pushbutton

momentary switch, and a separate shutdown control for

the linear regulator.

Step-Up Converter

During DC-DC converter operation, the internal N-chan-

nel MOSFET turns on for the first part of each cycle,

allowing current to ramp up in the inductor and store

energy in a magnetic field. During the second part of

each cycle, the MOSFET turns off and inductor current

flows through the synchronous rectifier to the output filter

capacitor and the load. As the energy stored in the

inductor is depleted, the current ramps down and the

synchronous rectifier turns off. The CLK/SEL pin deter-

mines whether a pulse-skipping or PWM control method

is used at light loads (Table 1).

Normal Operation

Pulling CLK/SEL low selects the MAX1765’s normal

operating mode. In this mode, the device operates in

PWM when driving medium to heavy loads and auto-

matically switches to SKIP mode if the load requires

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