MAX761/MAX762
In non-bootstrapped mode, the IC is powered from the
supply voltage, V
IN
, and operates with minimum supply
current. Since the voltage applied to the gate of the inter-
nal FET is reduced, efficiency declines with low input
voltages. Note: In non-bootstrapped mode, there is no
fixed-output operation; external resistors must be
used to set the output voltage. Use 1% external feed-
back resistors when operating in non-bootstrapped
mode (Figure 3).
Use bootstrapped mode when V
IN
is below approxi-
mately 4V. For V
IN
between 4V and 6V, the trade-off is
lower supply current in non-bootstrapped mode versus
higher output current in bootstrapped mode (see
Typical Operating Characteristics
).
Pulse-Frequency Modulation
(PFM) Control Scheme
The MAX761/MAX762 use a proprietary current-limited
PFM control scheme. This control scheme combines
the ultra-low supply current of pulse-skipping PFM con-
verters with the high full-load efficiency characteristic of
current-mode pulse-width-modulation (PWM) convert-
ers. It allows the devices to achieve high efficiency over
a wide range of loads, while the current-sense function
and high operating frequency allow the use of tiny
external components.
As with traditional PFM converters, the internal power
MOSFET is turned on when the voltage comparator
senses the output is out of regulation (Figure 1).
However, unlike traditional PFM converters, switching is
accomplished through the combination of a peak cur-
rent limit and a pair of one-shots that set the maximum
on-time (8µs) and minimum off-time (1.3µs) for the
switch. Once off, the minimum off-time one-shot holds
the switch off for 1.3µs. After this minimum time, the
switch either (1) stays off if the output is in regulation, or
(2) turns on again if the output is out of regulation.
The MAX761/MAX762 also limit the peak inductor cur-
rent, allowing the devices to run in continuous-conduc-
tion mode (CCM) and maintain high efficiency with
heavy loads (Figure 4a). This current-limiting feature is
a key component of the control circuitry. Once turned
on, the switch stays on until either (1) the maximum on-
time one-shot turns it off (8µs later), or (2) the current
limit is reached.
To increase light-load efficiency, the current limit for the
first two pulses is set to half the peak current limit. If
those pulses bring the output voltage into regulation,
the voltage comparator holds the MOSFET off, and the
current limit remains at half the peak current limit. If the
output voltage is still out of regulation after two pulses,
the current limit for the next pulse is raised to the full
current limit of 1A (Figure 4b).
Internal vs. External Resistors
When external feedback resistors are used, an internal
undervoltage lockout system prevents start-up until V+
rises to about 2.7V. When external feedback resistors are
12V/15V or Adjustable, High-Efficiency,
Low I
Q
, Step-Up DC-DC Converters
8 _______________________________________________________________________________________
Figure 2. Bootstrapped Operating Circuit
Figure 3. Non-Bootstrapped Operating Circuit
V
IN
=
+5V
LX
LBO
GND
MAX761
SHDN
V+
REF
L1
18µH
LBI
FB
D1
1N5817
C1
33µF
C3
0.1µF
C4
33µF
C2
0.1µF
R4
R3
100k
+12V at
150mA
LOW-BATTERY
OUTPUT
1
8
7
5
4
2
3
6
V
IN
LX
LBO
GND
MAX761
MAX762
SHDN
V+
REF
L1
18µH
LBI
FB
D1
1N5817
C1
C2
C4
R4
R3
100k
ADJUSTABLE
OUTPUT (V
OUT
)
LOW-BATTERY
DETECT OUTPUT
C3
R2
8
2
5
4
7
6
1
3
LOW-BATTERY
DETECT
R4 = R3
( )
V
TRIP
- V
REF
V
REF
R2 = R1 ( -1)
V
OUT
V
REF
V
REF
= 1.5V NOMINAL
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
R1
7
