MAX761ESA Maxim Integrated Products, MAX761ESA Datasheet - Page 8

• Manufacturer Part Number: MAX761ESA
• Description: IC DC/DC CONV STEP-UP 12V 8-SOIC
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX761ESA

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 12V, 5 ~ 16.5 V
• Current - Output: 150mA
• Frequency - Switching: 300kHz
• Voltage - Input: 2 ~ 16.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Power - Output: 471mW
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

12V/15V or Adjustable, High-Efficiency,
Low I
In non-bootstrapped mode, the IC is powered from the
supply voltage, V
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
mately 4V. For V
lower supply current in non-bootstrapped mode versus
higher output current in bootstrapped mode (see
Typical Operating Characteristics ).
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-
Figure 2. Bootstrapped Operating Circuit
8
V
+5V
R4
R3
IN
_______________________________________________________________________________________
=
C1
33µF
C3
0.1µF
Q
, Step-Up DC-DC Converters
IN
IN
5
4
2
3
Pulse-Frequency Modulation
, and operates with minimum supply
between 4V and 6V, the trade-off is
18µH
L1
SHDN
REF
LBI
FB
MAX761
GND
(PFM) Control Scheme
LX
7
6
LBO
1N5817
V+
D1
IN
8
1
is below approxi-
LOW-BATTERY
C2
0.1µF
OUTPUT
+12V at
150mA
C4
33µF
100k
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).
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
Figure 3. Non-Bootstrapped Operating Circuit
V
C1 = 33µF
C2 = 0.1µF
C3 = 0.1µF
C4 = 33µF
LOW-BATTERY
DETECT
R4 = R3
V
REF
R4
R3
IN
= 1.5V NOMINAL
(
V TRIP - V REF
C1
V REF
C3
C2
)
Internal vs. External Resistors
18µH
L1
8
2
5
4
V+
SHDN
LBI
REF
MAX761
MAX762
GND
LX
6
7
7
1N5817
D1
LBO
FB
C4
3
1
R2
R1
DETECT OUTPUT
OUTPUT (V
LOW-BATTERY
R2 = R1
ADJUSTABLE
(
V OUT
V REF
100k
OUT
)
-1
)