MAX770C Просмотр технического описания (PDF) - Maxim Integrated

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MAX770C

5V/12V/15V or Adjustable, High-Efficiency, Low IQ, Step-Up DC-DC Controllers

Maxim Integrated 

5V/12V/15V or Adjustable, High-Efficiency,

Low IQ, Step-Up DC-DC Controllers

Smaller capacitors are acceptable for light loads or in

applications that can tolerate higher output ripple.

Since the output filter capacitor’s ESR affects efficien-

cy, use low-ESR capacitors for best performance. The

smallest low-ESR surface-mount tantalum capacitors

currently available are the Sprague 595D series. Sanyo

OS-CON organic semiconductor through-hole capaci-

tors and the Nichicon PL series also exhibit low ESR.

See Table 2.

Input Bypass Capacitors

The input bypass capacitor (C1) reduces peak currents

drawn from the voltage source and also reduces noise

at the voltage source caused by the switching action of

the MAX770–MAX773. The input voltage source imped-

ance determines the size of the capacitor required at

the V+ input. As with the output filter capacitor, a low-

ESR capacitor is recommended. For output currents up

to 1A, 150µF (C1) is adequate, although smaller

bypass capacitors may also be acceptable.

Bypass the IC with a 0.1µF ceramic capacitor (C2)

placed close to the V+ and GND pins.

Reference Capacitor

Bypass REF with a 0.1µF capacitor (C3). REF can

source up to 100µA of current.

Setting the Low-Battery-Detector Voltage

To set the low-battery detector’s falling trip voltage

(VTRIP(falling)), select R3 between 10kΩ and 500kΩ

(Figure 9), and calculate R4 as follows:

( ) VTRIP - VREF

R4 = (R3) ———————

VREF

where VREF = 1.5V.

The rising trip voltage is higher because of the com-

parator’s approximately 20mV of hysteresis, and is

determined by:

VTRIP

(rising)

=

(VREF

+

20mV)

(1

+

—R—4 )

R3

Connect a high value resistor (larger than R3 + R4)

between LBI and LBO if additional hysteresis is required.

Connect a pull-up resistor (e.g., 100kΩ) between LBO

and V+. Tie LBI to GND and leave LBO floating if the

low-battery detector is not used.

VIN

V+

R4

MAX773

LBI

LBO

R3

R5

100k

LOW-BATTERY

OUTPUT

GND

( ) R4 = R3 VTRIP -1

VREF

VREF = 1.5V

Figure 9. Input Voltage Monitor Circuit

__________Applications Information

MAX773 Operation with High

Input/Output Voltages

The MAX773’s shunt regulator input allows high volt-

ages to be converted to very high voltages. Since the

MAX773 runs off the 6V shunt (bootstrapped operation

is not allowed), the IC will not see the high input volt-

age. Use an external logic-level N-FET as the power

switch, since only 6V of VGS are available. Also, make

sure all external components are rated for very high

output voltage. Figure 3e shows a circuit that converts

28V to 100V.

Low Input Voltage Operation

When using a power supply that decays with time

(such as a battery), the N-FET transistor will operate in

its linear region when the voltage at EXT approaches

the threshold voltage of the FET, dissipating excessive

power. Prolonged operation in this mode may damage

the FET. This effect is much more significant in non-

bootstrapped mode than in bootstrapped mode, since

bootstrapped mode typically provides much higher

VGS voltages. To avoid this condition, make sure VEXT

is above the VTH of the FET, or use a voltage detector

(such as the MAX8211) to put the IC in shutdown mode

once the input supply voltage falls below a predeter-

mined minimum value. Excessive loads with low input

voltages can also cause this condition.

18 ______________________________________________________________________________________

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