Rainbow Electronics MAX1790 - Manual (Page 6)

Detailed Description
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.
MAX1790
Low-Noise Step-Up DC-DC Converter
6 _______________________________________________________________________________________
Pin Description
Switch Pin. Connect the inductor/catch diode to LX and minimize the trace area for lowest EMI. LX 5
Supply Pin. Bypass IN with at least a 1µF ceramic capacitor directly to GND. IN 6
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.
FREQ 7
Soft-Start Control Pin. Connect a soft-start capacitor (C SS ) to this pin. Leave open for no soft-start. The soft-
start capacitor is charged with a constant current of 4µA. Full current limit is reached after t = 2.5 · 10 5 C SS .
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.
SS 8
Ground GND 4
Shutdown Control Input. Drive SHDN low to turn off the MAX1790. SHDN 3
PIN
Feedback Pin. Reference voltage is 1.24V nominal. Connect an external resistor-divider tap to FB and
minimize the trace area. Set V
OUT according to: V OUT = 1.24V (1 + R1 / R2). See Figure 1.
FB 2
Compensation Pin for Error Amplifier. Connect a series RC from COMP to ground. See the Loop
Compensation section for component selection guidelines.
COMP 1
FUNCTION NAME
LX
IN
V IN
2.6V TO 5.5V
GND
FREQ
V OUT
COMP
SS
SHDN
FB
R1
R2
L
0.027 µF
MAX1790
C OUT
C1
10
µF
10V
R COMP
C COMP
C COMP2
D1
C
IN
10 µF
6.3V
640kHz
1.2MHz
ON/OFF
MBRS130LT1
0.1 µF*
* OPTIONAL
V IN
Figure 1. Typical Application Circuit