Rainbow Electronics MAX863 User Manual
Page 6
MAX863
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
6
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The flip-flop resets and turns off the MOSFET when
either a) the voltage across the current-sense resistor
exceeds 100mV, or b) the 17.5µs maximum on-time
one-shot trips. When the MOSFET turns off, the mag-
netic field begins to collapse, and forces current into
the output capacitor and load. As the stored energy is
transferred to the output, the inductor current ramps
down. The output capacitor smoothes out the energy
transfer by storing charge when the diode current is
high, then supplying current to the load during the first
half of each cycle, maintaining a steady output voltage.
Resetting the flip-flop sets the off-time one-shot, dis-
abling the error-comparator output and forcing the
MOSFET off for at least 2µs to enforce a minimum time
for energy transfer to the output. The MAX863 waits
until the output voltage drops again before beginning
another cycle. The MAX863’s switching frequency
depends on the load current and input voltage.
Q
TRIG
MAX ON-TIME
ONE-SHOT
LOW-
VOLTAGE
OSCILLATOR
TIMING
BLOCK
BIAS
TIMING
BLOCK
EXT2
PGND
BOOT
EXT1
V
DD
ERROR
COMPARATOR
CURRENT-SENSE
COMPARATOR
TRIG
Q
MIN ON-TIME
ONE-SHOT
CURRENT-
SENSE
COMPARATOR
ERROR
COMPARATOR
100mV
S
Q
R
MAX863
UVLO
100mV
100mV
REF
V
DD
GND
SHDN2
SHDN1
REF
REF
1.25V
REF
FB2
CS2
CS1
SENSE1
FB1
LBO
LBI
V
DD
- 100mV
REF
N
N
Figure 1. Functional Diagram