High-Efficiency LCD Boost

with True Shutdown

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Detailed Description

The MAX8570 family of compact, step-up DC-DC con-

verters operate from a 2.7V to 5.5V supply. Consuming

only 25µA of supply current, these ICs include an inter-

nal MOSFET switch with a low on-resistance. A true-

shutdown feature disconnects the battery from the load

and reduces the supply current to 0.05µA (typ). These

DC-DC converters are available with either a fixed 15V

output or are adjustable up to 28V. Three current-limit

options are available: 110mA, 250mA, and 500mA. See

the Selector Guide on page 1.

Control Scheme

The MAX8570 family features a minimum off-time cur-

rent-limited control scheme operating in discontinuous

mode. An internal p-channel MOSFET switch connects

converter is operating. When the converter is shut

down, this switch disconnects the input supply from the

inductor (see Figure 1).

To boost the output voltage, an n-channel MOSFET

switch turns on and allows current to ramp up in the

inductor. Once this current reaches the current limit,

the switch turns off and the inductor current flows

through D1 to supply the output. The switching fre-

quency varies depending on the load and input voltage

and can be up to 800kHz.

Setting the Output Voltage

The output voltage of the MAX8570, MAX8571, and

resistor voltage-divider (see Figure 2). Select R2 from

10kΩ to 600kΩ and calculate R1 with the following

equation:

28V. For best accuracy, ensure that the bias current

through the feedback resistors is at least 2µA.

The MAX8572, MAX8573, and MAX8575 have a fixed

15V output. When using these parts, connect OUT

directly to the output (see Figure 3).

Shutdown (

SHDN)

Drive SHDN low to enter shutdown. During shutdown

the supply current drops to 0.05µA (typ), the output is

disconnected from the input, and LX enters a high-

impedance state. The capacitance and load at the out-

can be pulled as high as 6V regardless of the input and

output voltages.

With a typical step-up converter circuit, the output

remains connected to the input through the inductor

and output rectifier, holding the output voltage to one

and allowing the output to draw power from the input.

The MAX8570 family features true-shutdown mode, dis-

connecting the output from the input with an internal p-

channel MOSFET switch when shut down. This

eliminates power draw from the input during shutdown.

Soft-Start

The MAX8570 family uses two soft-start mechanisms.

When the true-shutdown feature is used (SW is con-

nected as in Figure 2 and Figure 3), the gate of the

internal high-side p-channel switch turns on slowly to

prevent inrush current. This takes approximately 200µs.

When SW is fully turned on, the internal n-channel

switch begins boosting the input to set the output volt-

of true shutdown), the minimum off-time of the internal

n-channel switch increases from 1µs to 5µs to control

inrush current.

Separate Power for Inductor

Separate power supplies can be used for the IC and

the inductor. This allows power to be used from a bat-

tery or supply with a voltage as low as 0.8V, or higher

using a separate inductor supply, SW is left unconnect-

ed and the supply is connected directly to the inductor

(see Figure 4). Note that in this configuration the output

is no longer disconnected from the input during shut-

down. In shutdown the output voltage goes to a diode

drop below the inductor supply voltage.

Protection Features

The MAX8570 family has protection features designed

to make it extremely robust to application errors (see

Table 1). If the output capacitor in the application is

missing, the MAX8570 family protects the internal

switch from being damaged. If the top feedback resis-

tor or the external diode is disconnected, the converter

stops switching and the output is resistively loaded to

ground. Similarly, if the external diode polarity is

reversed, the converter discontinues switching. If the

bottom feedback resistor is missing, the output stays at

a diode drop less than the inductor supply voltage or

1.226V (whichever is greater). In fact, in response to

most fault conditions, the MAX8570 family protects not

only itself, but also the downstream circuitry.

RR

V

V

OUT

FB

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1

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