Rainbow Electronics MAX776 - Manual (Page 9)

_______________Detailed Description
The MAX774/MAX775/MAX776 are negative-output,
inverting power controllers that can be configured to drive
an external P-channel MOSFET. The output voltages are
preset to -5V (MAX774), -12V (MAX775), or -15V
(MAX776). Additionally, all three parts can be set to any
desired output voltage using an external resistor divider.
The MAX774/MAX775/MAX776 have a unique control
scheme (Figure 1) that combines the advantage of
pulse-skipping, pulse-frequency-modulation (PFM)
converters (ultra-low supply current) with the advan-
tage of pulse-width-modulation (PWM) converters (high
efficiency with heavy loads). This control scheme
allows the devices to achieve 85% efficiency with loads
from 5mA to 1A.
As with traditional PFM converters, the external
P-channel MOSFET power transistor is turned on when
the voltage comparator senses that the output is below
the reference voltage. However, unlike traditional PFM
converters, switching is controlled by the combination
of a switch current limit (210mV/R
SENSE ) and
on-time/off-time limits set by one-shots. Once turned
on, the MOSFET stays on until:
1) the 16µs maximum on-time limit is reached
or
2) the switch current reaches its limit (as set by the
current-sense resistor).
Once off, the switch is typically held off for a minimum of
2.3µs. It will stay off until the output drops below the level
determined by V
REF and the feedback divider network.
MAX774/MAX775/MAX776
-5V/-12V/-15V or Adjustable,
High-Efficiency, Low I
Q
Inverting DC-DC Controllers
_______________________________________________________________________________________ 9
N
1.5V
REFERENCE
Q TRIG
ONE-SHOT
Q TRIG
ONE-SHOT
CURRENT
CONTROL CIRCUITS
MODE
COMPARATOR
ERROR
COMPARATOR
CURRENT
COMPARATOR
FROM V+
FROM OUT
0.2V
(FULL
CURRENT)
0.1V
(HALF
CURRENT)
FROM
V+
MAX774
MAX775
MAX776
REF
V+
SHDN
FB
OUT
CS
EXT
GND
S
R
Q
50mV
Figure 1. Block Diagram