1.4MHz SOT23 Current-Mode

Step-Up DC-DC Converter

6

_______________________________________________________________________________________

Detailed Description

The MAX1896 is a highly efficient power supply that

employs a current-mode, fixed-frequency pulse-width

modulation (PWM) architecture for fast-transient response

and low-noise operation. The functional diagram is shown

in Figure 2. As the load varies, the error amplifier sets the

inductor peak current necessary to supply the load and

regulate the output voltage. To maintain stability at high

duty cycle, a slope-compensation signal is internally

summed with the current-sense signal.

At light loads, this architecture allows the MAX1896 to

skip cycles to prevent overcharging the output voltage.

In this region of operation, the inductor ramps up to a

peak value of about 100mA, discharges to the output

and waits until another pulse is needed again.

Output-Current Capability

The output-current capability of the MAX1896 is a func-

tion of current limit, input voltage, and inductor value.

Because of the slope compensation used to stabilize

the feedback loop, the duty cycle affects the current

limit. The output-current capability is governed by the

following equation:

where:

Characteristics)

L = inductor value, (H)

η = conversion efficiency, 0.85 nominal

Soft-Start

The MAX1896 can be programmed for soft-start upon

power-up with an external capacitor. When the

charged at a constant current of 4µA, ramping up to

0.5V. During this time, the SS voltage directly controls

current is available after the soft-start cycle is complet-

ed. When the MAX1896 is turned off, the soft-start

capacitor is internally discharged to ground.

Shutdown

The MAX1896 shuts down to reduce the supply current

to 0.01µA when SHDN is low. In this mode, the internal

reference, error amplifier, comparators, biasing circuit,

and N-channel MOSFET are turned off. The step-up

converter’s output is still connected to IN via the exter-

nal inductor and output rectifier.

Applications Information

The MAX1896 operates well with a variety of external

components. The components in Figure 1 are suitable

for most applications. See the following sections to opti-

mize external components for a particular application.

Inductor Selection

Inductor selection depends on input voltage, output volt-

age, maximum current, size, and availability of inductor

values. Other factors can include efficiency and ripple

voltage. Inductors are specified by their inductance (L),

step-up circuit equations are useful in choosing the

inductor values based on the application. They allow the

trading of peak current and inductor value while consid-

ering component availability and cost.

The equation used here assumes a constant LIR, which

is the ratio of the inductor peak-to-peak AC current to

average DC inductor current. A good compromise

between the size of the inductor versus loss and output

ripple is to choose an LIR of 0.3 to 0.5. The peak induc-

tor current is then given by:

where:

I

Ix V

xV

x

LIR

PK

OUT MAX

OUT

IN MIN

()

()

=













+













η

1

2

DUTY

DUTY CYCLE

VV

V

VI

x R

V

OUT

IN

DIODE

OUT

LIM

ON

DIODE

==

+

+

−

−

I

I

Duty x V

f

xx

V

V

OUT MAX

LIM xx Duty

IN

OSC x L

IN

OUT

()

( .

.

))

(

.

=

×





























−−

145

0 9

05

η