MAX863 Maxim, MAX863 Datasheet - Page 7

• Manufacturer Part Number: MAX863
• Description: Dual / High-Efficiency / PFM / Step-Up DC-DC Controller
• Manufacturer: Maxim

Each converter in the MAX863 determines from moment
to moment whether to switch or not, waiting until the out-
put voltage drops before initiating another cycle. Under
light loads, the inductor current ramps to zero before the
next cycle; this is discontinuous-conduction mode.
Continuous-conduction mode occurs when the next
switching cycle begins while current is still flowing
through the inductor. The transition point between dis-
continuous- and continuous-conduction mode is deter-
mined by input and output voltages, and by the size of
the inductor relative to the peak switching current. In
general, reducing inductance toward the minimum rec-
ommended value pushes the transition point closer to
the maximum load current. If the inductor value is low
enough or the output/input voltage ratio high enough,
the DC-DC converter may remain in discontinuous-con-
duction mode throughout its entire load range.
Figure 2. Bootstrapped Typical Operating Circuit
Continuous/Discontinuous-Conduction
V
IN
= 1.5V TO THE LOWER OF V
220 F
0.1
V
10V
C1
_______________________________________________________________________________________
OUT1
= 5V
R5
R6
Dual, High-Efficiency, PFM, Step-Up
C2
0.1 F
OUT1
100k
R7
LOW-BATTERY
DETECTOR OUTPUT
OR V
MBRS340T3
OUT2
D1
Modes
L1
10 H
2A
R1
50m
N1A
SENSE1
CS1
LBI
EXT1
LBO
FB1
100 F
0.1
10V
C3
MAX863
The MAX863 transitions into continuous-conduction
mode in two ways, depending on whether preset or
adjustable mode is used and how the external feed-
back network is compensated. Under light loads, the IC
switches in single pulses (Figure 3a). The threshold of
transition into continuous-conduction mode is reached
when the inductor current waveforms are adjacent to
one another, as shown in Figure 3b. As the load
increases, the transition into continuous-conduction
mode progresses by raising the minimum inductor cur-
rent (Figures 3c, 3d). Depending on feedback compen-
sation, transition into continuous-conduction mode may
also progress with grouped pulses (Figures 3e, 3f).
Pulse groups should be separated by less than two or
three switching cycles. Output ripple should not be
significantly more than the single-cycle no-load case.
PGND
V
DD
100 F
SHDN2
SHDN1
0.1
BOOT
EXT2
GND
10V
CS2
REF
C4
FB2
50m
DC-DC Controller
R2
ON/OFF
N1B
IRF7301
L2
10 H
2A
C7
0.1 F
MBRS340T3
D2
165k
1%
R3
R4
100k
1%
V
C6
10pF
OUT2
= 3.3V
C5
330 F
10V
0.1
7