Active Rectifier

The internal active rectifier of the MAX877/MAX878/

MAX879 replaces the external Schottky catch diode in

normal boost operation. The rectifier consists of a PNP

pass transistor and a unique control circuit which, in

shutdown mode, entirely disconnects the load from the

source. This is a distinct advantage over standard boost

topologies, since it prevents battery drain in shutdown.

The MAX877/MAX878/MAX879 can withstand a momen-

tary short at the output in normal operation.

The active rectifier also acts as a zero-dropout regulator

if the input exceeds the regulated output.The device still

switches to deliver power to the output, and the differ-

ence between the input and output voltage appears

across the rectifier. Efficiency is similar to that of a linear

regulator if the MAX877/MAX878/MAX879 are used as

step-down converters. The maximum output current

approximated by:

≈

Shutdown

Shutdown (S

—

H

—

D

—

N

–

) is a high-impedance, active-low

input. Connect it to IN for normal operation. Keeping

S

—

H

—

D

—

N

–

at ground holds the converters in shutdown

mode. Since the active rectifier is turned off in this

mode, the path from input to load is cut, and the output

effectively drops to 0V. The supply current in shutdown

+ 250mV. When S

—

H

—

D

—

N

–

is below this threshold, the

device is shut down; it is enabled with S

—

H

—

D

—

N

–

above the

threshold. When driven from external logic, S

—

H

—

D

—

N

–

can

Current Limit

Connecting ILIM to IN sets an LX current limit of 1A. For

smaller output power levels that do not require the maxi-

mum peak current, reduce the peak inductor current by

connecting a resistor between ILIM and IN. This opti-

mizes overall efficiency and allows very small, low-cost

coils with lower current ratings. See Figure 2 to select

the resistor (see also

Inductor Selection section).

Output Voltage Selection

The MAX877’s output voltage is fixed at 5V. The MAX878’s

output voltage can be set to 3V by leaving the SEL pin

open, or to 3.3V by connecting SEL to AGND.

The MAX879’s output voltage is set by two resistors, R1

and R2 (Figure 3), which form a voltage divider

between the output and the FB pin. The output voltage

can be set from 2.5V to 6.0V by the equation:

(R1 + R2)

R2

To simplify the resistor selection:

R1 = R2

(

- 1

)

Since the input current at FB has a maximum of 40nA,

large values (10k

Ω to 50kΩ for R2) can be used without

significant accuracy loss. For 1% error, the current

through R2 should be at least 100 times FB’s bias current.

When large values are used for the feedback resistors

(R1 > 50k

Ω), stray output impedance at FB can add a

“lag” to the feedback response, destabilizing the regula-

tor and creating a larger ripple at the output. Lead

lengths and circuit board traces at the FB node should be

kept short. Reduce ripple by adding a “lead” compensa-

tion capacitor (C3, 100pF to 50nF) in parallel with R1.

5V/3.3V/3V/Adjustable-Output,

Step-Up/Step-Down DC-DC Converters

8

_______________________________________________________________________________________

1200

02468

10

12

14

CURRENT-LIMIT RESISTOR

vs. PEAK INDUCTOR CURRENT

RESISTOR VALUE (k

Ω)

1000

600

800

400

200

0

Figure 2. Current-Limit Resistor vs. Peak Inductor Current