5V or Adjustable, Low-Voltage,

Step-Up DC-DC Controller

8

_______________________________________________________________________________________

Unlike traditional PFM converters, the MAX608 uses a

sense resistor to control the peak inductor current. The

device also operates with high switching frequencies

(up to 300kHz), allowing the use of miniature external

components.

As with traditional PFM converters, the power transistor

is not turned on until the voltage comparator senses

the output is out of regulation. However, unlike tradition-

al PFM converters, the MAX608 switch uses the combi-

nation of a peak current limit and a pair of one-shots

that set the maximum on-time (16µs) and minimum off-

time (2.3µs); there is no oscillator. Once off, the mini-

mum off-time one-shot holds the switch off for 2.3µs.

After this minimum time, the switch either 1) stays off if

the output is in regulation, or 2) turns on again if the

output is out of regulation.

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 MAX608 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 5V output at 500mA from

a 2V input, only 75mV of output ripple occurs, using the

circuit of Figure 2a.

Low-Voltage Start-Up Oscillator

The MAX608 features a low input voltage start-up oscil-

lator that guarantees start-up with no load for input volt-

ages down to 1.8V. At these low voltages, the output

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 MAX608 disregards

the error-comparator output when the output voltage is

less than 2.5V. Above 2.5V, the error-comparator and

normal one-shot timing circuitry are used.

Shutdown Mode

When SHDN is high, the MAX608 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

The MAX608’s output voltage is preset to 5V (FB = 0V),

or it can be adjusted from 16.5V down to 3V using exter-

nal 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)

OUT must always be connected to the circuit output.

Figure 2 shows various circuit configurations for preset/

adjustable operation.

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

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

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 out-

put-current capability will be much less when the induc-

tance value is too low, because the IC will always operate

in discontinuous mode. If the inductor value is too low, the

MAX608

R1

R2

C5*

GND

FB

R1 = 10k TO 500k

* OPTIONAL, SEE TEXT FOR VALUE

R2 = R1

(

-1

)

Figure 3. Adjustable Output Circuit