MAX761CSA Maxim Integrated Products, MAX761CSA Datasheet - Page 9

• Manufacturer Part Number: MAX761CSA
• Description: IC DC/DC CONV STEP-UP 12V 8-SOIC
• Manufacturer: Maxim Integrated Products
• Type: Step-Up (Boost)r

Specifications of MAX761CSA

• 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: 0°C ~ 70°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

used in a bootstrapped circuit (Figure 5), undervoltage
lockout prevents start-up at low input voltages; but
once started, operation can continue down to a lower
voltage that depends on the load.
There is no undervoltage lockout when the internal feed-
back resistors are used (Figure 2), and special circuitry
guarantees start-up at 2.0V. The start-up circuitry fixes
the duty cycle at 50% until V+ is driven to 2.5V, above
which the normal control system takes over.
The MAX761/MAX762 enter shutdown mode when
SHDN is high. In this mode, the internal biasing circuitry
is turned off (including the reference) and V
V+ minus a diode drop (due to the DC path from the
input to the output). In shutdown mode, the supply cur-
rent drops to less than 5µA. SHDN is a TTL/CMOS logic
level input. Connect SHDN to GND for normal operation.
LBO is high impedance during shutdown.
When delivering high output currents, the MAX761/
MAX762 operate in CCM. In this mode, current always
flows in the inductor, and the control circuit adjusts the
switch’s duty cycle on a cycle-by-cycle basis to maintain
regulation without exceeding the switch-current capabili-
ty. This provides excellent load-transient response and
high efficiency.
In discontinuous-conduction mode (DCM), current
through the inductor starts at zero, rises to a peak value,
then ramps down to zero on each cycle. Although effi-
ciency is still excellent, the switch waveforms contain
Figure 4a. CCM, Heavy Load Current Waveform (500mA/div)
_______________________________________________________________________________________
12V/15V or Adjustable, High-Efficiency,
Modes of Operation
Low I
Shutdown Mode
OUT
1A
500mA
Q
equals
, Step-Up DC-DC Converters
ringing (the inductor's self-resonant frequency). This
ringing is normal and poses no operational problems.
The MAX761/MAX762 provide a low-battery comparator
that compares the voltage on LBI to the 1.5V reference
voltage. When the LBI voltage is below V
open-drain output) goes low. The low-battery compara-
tor’s 20mV of hysteresis adds noise immunity, prevent-
ing repeated triggering of LBO. Use a resistor-divider
network between V+, LBI, and GND to set the desired
trip voltage V
ignored and LBO is high impedance. The value of
resistor R3 should be no larger than 500kΩ to ensure
the LBI leakage current does not cause inaccuracies in
V
The MAX761/MAX762’s output voltage can be adjusted
from 5V to 16.5V using external resistors R1 and R2
configured as shown in Figures 3 and 5. For adjustable-
output operation, select feedback resistor R1 in the
10kΩ to 250kΩ range. Higher R1 values within this
range give lowest supply current and best light-load
efficiency. R2 is given by:
where V
Note: Tie FB to GND for fixed-output operation
(bootstrapped mode only).
Figure 4b. Light/Medium Load Current Waveform (500mA/div)
__________________Design Procedure
TRIP
.
REF
R2 = (R1)( V
= 1.5V.
TRIP
(Figure 3). When SHDN is high, LBI is
Setting the Output Voltage
V
OUT
REF
Low-Battery Detector
- 1)
REF
, LBO (an
500mA
1A
0A
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