Datasheet

MAX1771
12V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Controller
_______________________________________________________________________________________
7
_______________Detailed Description
The MAX1771 is a BiCMOS, step-up, switch-mode pow-
er-supply controller that provides a preset 12V output,
in addition to adjustable-output operation. Its unique
control scheme combines the advantages of pulse-fre-
quency modulation (low supply current) and pulse-
width modulation (high efficiency with heavy loads),
providing high efficiency over a wide output current
range, as well as increased output current capability
over previous PFM devices. In addition, the external
sense resistor and power transistor allow the user to tai-
lor the output current capability for each application.
Figure 1 shows the MAX1771 functional diagram.
The MAX1771 offers three main improvements over
prior pulse-skipping control solutions: 1) the converter
operates with miniature (5mm height and less than
9mm diameter) surface-mount inductors due to its
300kHz switching frequency; 2) the current-limited PFM
control scheme allows 90% efficiencies over a wide
range of load currents; and 3) the maximum supply
current is only 110µA.
The device has a shutdown mode that reduces the
supply current to 5µA max.
Bootstrapped/Non-Bootstrapped Modes
Figure 2 shows the standard application circuits for
bootstrapped and non-bootstrapped modes. In boot-
strapped mode, the IC is powered from the output
(V
OUT
, which is connected to V+) and the input voltage
range is 2V to V
OUT
. The voltage applied to the gate of
the external power transistor is switched from V
OUT
to
ground, providing more switch gate drive and thus
reducing the transistor’s on-resistance.
In non-bootstrapped mode, the IC is powered from the
input voltage (V+) and operates with minimum supply
current. In this mode, FB is the output voltage sense
point. Since the voltage swing applied to the gate of the
external power transistor is reduced (the gate swings
from V+ to ground), the power transistor’s on-resistance
1.5V
REFERENCE
Q TRIG
QS
F/F
R
QTRIG
LOW-VOLTAGE
OSCILLATOR
2.5V
0.1V
MAX ON-TIME
ONE-SHOT
MIN OFF-TIME
ONE-SHOT
CURRENT-SENSE
AMPLIFIER
DUAL-MODE
COMPARATOR
FB
REF
50mV
ERROR
COMPARATOR
SHDN
V+
EXT
CS
BIAS
CIRCUITRY
N
MAX1771
2.3µs
16µs
Figure 1. Functional Diagram