The control circuitry allows the IC to operate in continu-

ous-conduction mode (CCM) while maintaining high

efficiency with heavy loads. When the power switch is

turned on, it stays on until either 1) the maximum on-

time one-shot turns it off (typically 16µs later), or 2) the

switch current reaches the peak current limit set by the

current-sense resistor.

The MAX1771 switching frequency is variable (depend-

ing on load current and input voltage), causing variable

switching noise. However, the subharmonic noise gen-

erated does not exceed the peak current limit times the

filter capacitor equivalent series resistance (ESR). For

example, when generating a 12V output at 500mA from

a 5V input, only 100mV of output ripple occurs using

the circuit of Figure 2a.

Low-Voltage Start-Up Oscillator

The MAX1771 features a low input voltage start-up oscil-

lator that guarantees start-up with no load down to 2V

when operating in bootstrapped mode and using inter-

nal feedback resistors. At these low voltages, the supply

voltage is not large enough for proper error-comparator

operation and internal biasing. The start-up oscillator

has a fixed 50% duty cycle and the MAX1771 disre-

gards the error-comparator output when the supply volt-

age is less than 2.5V. Above 2.5V, the error-comparator

and normal one-shot timing circuitry are used. The low-

voltage start-up circuitry is disabled if non-bootstrapped

mode is selected (FB is not tied to ground).

Shutdown Mode

When SHDN is high, the MAX1771 enters shutdown

mode. In this mode, the internal biasing circuitry is

input to the output). In shutdown mode, the supply

current drops to less than 5µA. SHDN is a TTL/CMOS

logic-level input. Connect SHDN to GND for normal

operation.

__________________Design Procedure

Setting the Output Voltage

To set the output voltage, first determine the mode of

operation, either bootstrapped or non-bootstrapped.

Bootstrapped mode provides more output current

capability, while non-bootstrapped mode reduces the

supply current (see

Typical Operating Characteristics).

If a decaying voltage source (such as a battery) is

used, see the additional notes in the

Low Input Voltage

Operation section.

The MAX1771’s output voltage can be adjusted from

very high voltages down to 3V, using external resistors

R1 and R2 configured as shown in Figure 3. For

adjustable-output operation, select feedback resistor

R1 in the 10k

Ω to 500kΩ range. R2 is given by:

R2 = (R1)

For preset-output operation, tie FB to GND (this

forces bootstrapped-mode operation.

Figure 2 shows various circuit configurations for boot-

strapped/non-bootstrapped, preset/adjustable operation.

Determining RSENSE

Use the theoretical output current curves shown in

using the minimum (worst-case) current-limit compara-

tor threshold value over the extended temperature

range (-40°C to +85°C). No tolerance was included for

assumed to be 0.5V, and the drop across the power

0.3V.

Determining the Inductor (L)

Practical inductor values range from 10µH to 300µH.

22µH is a good choice for most applications. In appli-

cations with large input/output differentials, the IC’s

output current capability will be much less when the

inductance value is too low, because the IC will always

operate in discontinuous mode. If the inductor value

is too low, the current will ramp up to a high level before

the current-limit comparator can turn off the switch.

12V or Adjustable, High-Efficiency,

_______________________________________________________________________________________

9

MAX1771

R1

R2

C5*

GND

FB

R1 = 10k TO 500k

* SEE TEXT FOR VALUE

R2 = R1

(

-1

)

Figure 3. Adjustable Output Circuit