Rainbow Electronics MAX1643 - Manual (Page 7)

MAX1642/MAX1643
High-Efficiency, Step-Up
DC-DC Converters for 1V Inputs
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BATT
PFI
0.5REF
PFO
GND
OUT
REF
N
P
OUT 1.7V
FB
SHDN
LX
RFRDY
REF
REF
0.5REF
START-UP
OSCILLATOR
TIMING
TON
EN
PDRV
NDRV
TOFF
LOGIC
MAX1642
Figure 1. MAX1642 Functional Diagram
When the error comparator detects that the output
voltage is too low, it turns on the internal N-channel
MOSFET switch until the on-time is satisfied (see Figure
1 and the Standard Application Circuits, Figures 2 and
3). During the on-time, current ramps up in the induc-
tor, storing energy in a magnetic field. When the MOS-
FET turns off, during the second half of each cycle, the
magnetic field collapses, causing the inductor voltage
to force current through the synchronous rectifier,
transferring the stored energy to the output filter capac-
itor and load. The output filter capacitor stores charge
while current from the inductor is high, then holds up
the output voltage until the second half of the next
switching cycle, smoothing power flow to the load.
Bootstrap DC-DC Block
The bootstrap block contains a low-voltage start-up
oscillator. This oscillator pumps up the output voltage
to approximately 1.7V, where the main DC-DC con-
verter can operate. The oscillator is powered from the
BATT input and drives an NPN switch. During start-up,
the P-channel synchronous rectifier remains off and
either its body diode or an external diode is used as an
output rectifier. Reduce the load as needed to allow
start-up with input voltages below 2V (see
Typical
Operating Characteristics ).
Shutdown (MAX1642)
Pulling SHDN low places the MAX1642 in shutdown
mode (I
SHDN = 2µA typical). In shutdown, the internal
switching MOSFET turns off, PFO goes high-
impedance, and the synchronous rectifier turns off to
prevent reverse current from flowing from the output
back to the input. However, there is still a forward cur-
rent path through the synchronous-rectifier body diode
from the input to the output. Thus, in shutdown, the out-
put remains one diode drop below the battery voltage
(V
BATT ). To disable the shutdown feature, connect
SHDN (a logic input) to BATT.