3.3V/5V or Adjustable,
Step-Up/Down DC-DC Converters
Configuration 3: High Efficiency, 11V, V
BATT
< 6.5V
In this configuration, N/E is driven high by LBO when
V
IN
> 6.5V (Figure 2b). When V
IN
< V
OUT
, the IC
boosts, and the linear regulator operates as a switch,
with minimum forward drop. When V
IN
> V
OUT
, linear
regulation begins. When V
IN
> 6.5V (set by R5 and R6),
the linear regulator forces a minimum forward drop of
V
FV
(typically 0.5V at 5V V
OUT
) as LBO drives N/E high.
This transition is not seen at the output, since the linear
regulator already has an input-output voltage difference
of 6.5V - 5V. Efficiency with V
IN
slightly higher than
V
OUT
is equal to that of configuration 1, so configura-
tion 3 is most suitable when the battery voltage may be
near V
OUT
. This hookup has no functional shortcomings
compared with configuration 2, except that two addi-
tional resistors (R5 and R6) are needed.
Configuration 4: Low Noise
With N/E connected to PS, when the IC is boosting, the
linear regulator operates with V
FV
forward voltage (typi-
cally 0.5V at 5V V
OUT
) for optimum noise rejection.
Linear regulation occurs when V
IN
> V
OUT
+ V
FV
. The
V
FV
voltage differential results in boost efficiency typi-
cally 10% lower than with the high-efficiency configura-
tions.
ILIM
The current-limit-select input, ILIM, selects between the
two peak current limits: 1.5A (ILIM = GND) and 0.8A
(ILIM = PS). If the application requires 200mA or less
from the MAX710/MAX711, select 0.8A. The lower peak
current limit permits the use of smaller, low-cost induc-
tors. The ILIM input is internally diode clamped to GND
and PS, and should not be connected to signals out-
side this range.
Shutdown and Standby Modes
Grounding SHDN turns off the MAX710/MAX711 com-
pletely, disconnecting the input from the output. Tie
SHDN to PS for normal operation.
The MAX710/MAX711 have a standby mode that shuts
down the step-up converter. The linear regulator
remains on with a 7µA (typ) LDO quiescent current.
Connect STBY to ground to enter standby mode; other-
wise, connect STBY to PS.
__________________Design Procedure
Output Voltage Selection
For the MAX710, you can obtain a 3.3V or 5V output
voltage by tying 3/5 to GND or PS. Efficiency is typically
85% over a 2mA to 250mA load range. The device is
bootstrapped, with power derived from the step-up
voltage output (at PS). Under all load conditions, the
MAX710/MAX711 typically start up with a 1V input. If
the battery voltage exceeds the programmed output
voltage, the output will linear regulate down to the
selected output voltage.
The MAX711’s adjustable output voltage is set by two
resistors, R1 and R2 (Figure 3), which form a voltage
divider between the output and FB. Use the following
equation to determine the resistor values:
R1 = R2 [(V
OUT
/ V
REF
) - 1]
where V
REF
= 1.25V.
Since the input bias current at FB has a maximum value
of 50nA, R1 and R2 can be large with no significant
accuracy loss. Choose R2 in the 100kΩ to 1MΩ range
and calculate R1 using the formula above. For 1%
error, the current through R1 should be at least 100
times FB’s bias current.
Low-Battery Comparator
The MAX710/MAX711 contain a comparator for low-
battery detection. If the voltage at LBI+ falls below that
at LBI- (typically connected to REF), LBO goes low.
Hysteresis is typically 50mV. Set the low-battery moni-
tor’s threshold with two resistors, R3 and R4 (Figure 2),
using the following equation:
R3 = R4 [(V
LBT
/ V
LBI
-) - 1]
