MAX863 Maxim, MAX863 Datasheet - Page 6

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

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
Dual, High-Efficiency, PFM, Step-Up
DC-DC Controller
Figure 1. Functional Diagram
6
SENSE1
_______________________________________________________________________________________
CS2
CS1
LBO
FB2
FB1
LBI
REF
100mV
V
DD
- 100mV
100mV
REF
COMPARATOR
CURRENT-SENSE
COMPARATOR
N
ERROR
N
TRIG
MIN ON-TIME
ONE-SHOT
100mV
REF
GND
V
DD
Q
Q
MAX ON-TIME
COMPARATOR
ONE-SHOT
CURRENT-
COMPARATOR
1.25V
REF
SENSE
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.
ERROR
TRIG
S
REF
R
Q
OSCILLATOR
VOLTAGE
LOW-
TIMING
BLOCK
TIMING
BLOCK
BIAS
SHDN2
SHDN1
MAX863
UVLO
EXT2
PGND
BOOT
EXT1
V
DD