Rainbow Electronics MAX1677 - Manual (Page 10)

MAX1677
Compact, High-Efficiency, Dual-Output
Step-Up and LCD Bias DC-DC Converter
10 ______________________________________________________________________________________
output voltage. In PWM mode, the MBC can supply up
to 350mA. Switching harmonics generated by the fixed-
frequency operation are consistent and easily filtered.
During PWM operation, the rising edge of the internal
clock sets a flip-flop, which turns on the N-channel
MOSFET (Figure 4). The switch turns off when the sum
of the voltage-error, slope-compensation, and current-
feedback signals trips the multi-input comparator and
resets the flip-flop; the switch remains off for the rest of
the cycle. Changes in the output voltage error signal
shift the inductor current level and modulate the MOS-
FET pulse width.
Clock-Synchronized PWM
The MAX1677 operates as a clock-synchronized cur-
rent-mode PWM when a clock signal (200kHz to
400kHz) is applied to CLK/SEL. This allows switching
harmonics to be positioned to avoid sensitive frequen-
cy bands, such as those near IF frequencies in wireless
applications.
Low Power PFM Mode
Pulling CLK/SEL low places the MAX1677 in low-power
standby mode. During standby mode, PFM operation
regulates the output voltage by transferring a fixed
amount of energy during each cycle, and then modulat-
ing the switching frequency to control the power deliv-
ered to the output. The device switches only as needed
to service the load, resulting in the highest possible effi-
ciency at light loads and an operating current of only
20µA. The MBC can supply up to 170mA when in PFM
mode (Table 1).
MAX1677
OUT
R3
10 Ω
C4
0.1 µF
POUT
LX
LCDLX
REF
FB
R2
R1
C5
0.1
µF
C1
100
µF
C2
100
µF
V IN
3.3V MAIN
BOOST OUTPUT
L2
10 µH
L1
10 µH
C3
4.7
µF
LCD BOOST OUTPUT
D2
MBR0530
LCDGND
LCDFB
GND
LCDON
CLK/SEL
ON
LCDPOL
PGND
Figure 2. LCD Converter in Positive Mode
MAX1677
OUT
R3
10 Ω
C4
0.1 µF
POUT
LX
LCDLX
REF
FB
C5
0.1
µF
C6
0.1
µF
R2 R1
C1
100
µF
C2
100
µF
V IN
3.3V MAIN
BOOST OUTPUT
L2
10 µH
L1
10 µH
C3
4.7
µF
-LCD BOOST
OUTPUT
D2
MBR0530
D3
MBR0530
LCDGND
LCDFB
GND
LCDON
CLK/SEL
ON
LCDPOL
PGND
Figure 3. LCD Converter in Negative Mode
R
S
Q
OSC
PWM-MODE
CURRENT-
LIMIT LEVEL
REF
FEEDBACK
POUT
LX
PGND
P
N
Figure 4. Controller Block Diagram in PWM Mode