MAX1790 Maxim, MAX1790 Datasheet - Page 6

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

Low-Noise Step-Up DC-DC Converter
The MAX1790 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 device regu-
lates the output voltage through a combination of an
error amplifier, two comparators, and several signal
generators (Figure 2). The error amplifier compares
the signal at FB to 1.24V and varies the COMP output.
The voltage at COMP determines the current trip point
each time the internal MOSFET turns on. As the load
varies, the error amplifier sources or sinks current to the
COMP output accordingly to produce the inductor peak
current necessary to service the load. To maintain sta-
bility at high duty cycle, a slope compensation signal is
summed with the current-sense signal.
At light loads, this architecture allows the MAX1790 to
“skip” cycles to prevent overcharging the output volt-
age. In this region of operation, the inductor ramps up
to a peak value of about 50mA, discharges to the out-
put, and waits until another pulse is needed again.
6
PIN
_______________________________________________________________________________________
1
2
3
4
5
6
7
8
NAME
COMP
SHDN
FREQ
GND
FB
LX
SS
IN
Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
Feedback Pin. Reference voltage is 1.24V nominal. Connect an external resistor-divider tap to FB and
minimize the trace area. Set V
Shutdown Control Input. Drive SHDN low to turn off the MAX1790.
Ground
Switch Pin. Connect the inductor/catch diode to LX and minimize the trace area for lowest EMI.
Supply Pin. Bypass IN with at least a 1µF ceramic capacitor directly to GND.
Frequency Select Input. When FREQ is low, the oscillator frequency is set to 640kHz. When FREQ is high,
the frequency is 1.2MHz. This input has a 5µA pull-down current.
Soft-Start Control Pin. Connect a soft-start capacitor (C
start capacitor is charged with a constant current of 4µA. Full current limit is reached after t = 2.5
The soft-start capacitor is discharged to ground when SHDN is low. When SHDN goes high, the soft-start
capacitor is charged to 0.5V, after which soft-start begins.
Detailed Description
OUT
according to: V
Figure 1. Typical Application Circuit
1.2MHz
640kHz
FUNCTION
0.027µF
OUT
ON/OFF
V
V
2.6V TO 5.5V
IN
= 1.24V (1 + R1 / R2). See Figure 1.
IN
SS
C
COMP2
) to this pin. Leave open for no soft-start. The soft-
SHDN
FREQ
SS
MAX1790
COMP
IN
R
C
GND
COMP
COMP
LX
FB
L
Pin Description
MBRS130LT1
0.1µF*
10µF
R2
C
D1
IN
6.3V
C1
10µF
10V
* OPTIONAL
R1
·
10
5
C
C
OUT
SS
V
OUT
.