Rainbow Electronics MAX8715 - Manual (Page 8)

MAX1790/MAX8715
Low-Noise Step-Up DC-DC Converters
8 _______________________________________________________________________________________
Detailed Description
The MAX1790/MAX8715 are highly efficient power sup-
plies that employ a current-mode, fixed-frequency
PWM architecture for fast transient response and low-
noise operation. The device regulates the output volt-
age 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 according-
ly to produce the inductor peak current necessary to ser-
vice the load. To maintain stability at high duty cycle, a
slope-compensation signal is summed with the current-
sense signal.
At light loads, this architecture allows the ICs to “skip”
cycles to prevent overcharging the output voltage. In
this region of operation, the inductor ramps up to a
fixed peak value (approximately 50mA, MAX1790 or
75mA, MAX8715), discharges to the output, and waits
until another pulse is needed again.
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 pulldown 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 x 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/MAX8715. 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
MAX8715
C
OUT
C1
10
µF
6.3V
R COMP
C COMP
C
COMP2
D1
C
IN
640kHz
1.2MHz
ON/OFF
MBRS130LT1
0.1 µF*
* OPTIONAL
V
IN
Figure 1. Typical Application Circuit