MAX1790 Maxim, MAX1790 Datasheet - Page 7

• Manufacturer Part Number: MAX1790
• Description: Low-Noise Step-Up DC-DC Converter
• Manufacturer: Maxim

The output current capability of the MAX1790 is a func-
tion of current limit, input voltage, operating frequency,
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:
I
Characteristics)
Duty = duty cycle = (V
(V
V
η =conversion efficiency, 85% nominal
The MAX1790 can be programmed for soft-start upon
power-up with an external capacitor. When the shutdown
pin is taken high, the soft-start capacitor (C
ately charged to 0.5V. Then the capacitor is charged at a
constant current of 4µA (typ). During this time, the SS
voltage directly controls the peak inductor current, allow-
ing 0A at V
The maximum load current is available after the soft-start
Figure 2. Functional Diagram
LIM
DIODE
OUT
= current limit specified at 65% (see Electrical
- I
0.5 · Duty · V
= catch diode forward voltage at I
LIM
I
OUT(MAX)
SS
· R
COMP
= 0.5V to the full current limit at V
SHDN
FREQ
ON
FB
_______________________________________________________________________________________
+ V
= [I
IN
DIODE
/ (f
OUT
Output Current Capability
LIM
OSC
5µA
· (1.26 - 0.4 · Duty) -
- V
Low-Noise Step-Up DC-DC Converter
OSCILLATOR
)
· L)] · η · V
IN
1.24V
+ V
DIODE
IN
ERROR
AMPLIFIER
COMPEN-
LIM
SS
SATION
SLOPE
) /
/ V
Soft-Start
∞
) is immedi-
OUT
SS
BIAS
= 1.5V.
Σ
ERROR
COMPARATOR
ENABLE
COMPARATOR
cycle is completed. When the shutdown pin is taken low,
the soft-start capacitor is discharged to ground.
The MAX1790’s frequency can be user selected to
operate at either 640kHz or 1.2MHz. Tie FREQ to GND
for 640kHz operation. For a 1.2MHz switching frequen-
cy, tie FREQ to IN. This allows the use of small, mini-
mum-height external components while maintaining low
output noise. FREQ has an internal pull-down, allowing
the user the option of leaving FREQ unconnected for
640kHz operation.
The MAX1790 shuts down to reduce the supply current
to 0.1µA when SHDN is low. In this mode, the internal
reference, error amplifier, comparators, and biasing cir-
cuitry turn off while the N-channel MOSFET is turned
off. The boost converter’s output is connected to IN via
the external inductor and catch diode.
Boost DC-DC converters using the MAX1790 can be
designed by performing simple calculations for a first
iteration. All designs should be prototyped and tested
prior to production. Table 1 provides a list of compo-
nents for a range of standard applications. Table 2 lists
component suppliers.
CLOCK
AND DRIVER
ENABLE
CONTROL
CURRENT
LOGIC
SENSE
Applications Information
MAX1790
START
SOFT-
Frequency Selection
4µA
N
IN
SS
LX
GND
Shutdown
7