High-Efficiency, Low-Supply-Current,
Compact, Step-Up DC-DC Converters
MAX1674/MAX1675/MAX1676
As a result, the synchronous rectifier significantly
improves efficiency without the addition of an external
component. Conversion efficiency can be as high as
94%, as shown in the
Typical Operating Characteristics.
For low-voltage inputs from single cells (Alkaline, NiCd,
or NiMH), use an external Schottky diode such as the
1N5817 to ensure start-up.
V
IN
R1
200Ω
BATT
22µH
Voltage Reference
The voltage at REF is nominally +1.30V. REF can
source up to 100µA to external circuits. The reference
maintains excellent load regulation (see
Typical Oper-
ating Characteristics).
A bypass capacitor of 0.1µF is
required for proper operation.
MAX1676
DAMPING
SWITCH
LX
OUT
0.1µF
47µF
V
OUT
Shutdown
The device enters shutdown when V
SHDN
is low.
During shutdown, the body diode of the P-channel
MOSFET allows current flow from the battery to the out-
put. V
OUT
falls to approximately V
IN
- 0.6V and LX
remains high impedance. The capacitance and load at
OUT determine the rate at which V
OUT
decays.
Shutdown can be pulled as high as 6V, regardless of
the voltage at OUT.
Figure 2. Simplified Diagram of Inductor Damping Switch
Current Limit Select Pin (MAX1676)
The MAX1676 allows a selectable inductor current limit
of either 0.5A or 1A. This allows flexibility in designing
for higher current applications or for smaller, compact
designs. Connect CLSEL to OUT for 1A or to GND for
0.5A. CLSEL draws 1.4µA when connected to OUT.
V
LX
1V/div
BATT/Damping Switch (MAX1676)
The MAX1676 is designed with an internal damping
switch to minimize ringing at LX. The damping switch
connects an external resistor (R1) across the inductor
when the inductor’s energy is depleted (Figure 2).
Normally, when the energy in the inductor is insufficient
to supply current to the output, the capacitance and
inductance at LX form a resonant circuit that causes
ringing. The ringing continues until the energy is dissi-
pated through the series resistance of the inductor. The
damping switch supplies a path to quickly dissipate this
energy, minimizing the ringing at LX. Damping LX ring-
ing does not reduce V
OUT
ripple, but does reduce EMI.
R1 = 200Ω works well for most applications while reduc-
ing efficiency by only 1%. Larger R1 values provide less
damping, but have less impact on efficiency. Generally,
lower values of R1 are needed to fully damp LX when
the V
OUT
/V
IN
ratio is high (Figures 2, 3, and 4).
2µs/div
Figure 3. LX Ringing Without Damping Switch
V
LX
1V/div
2µs/div
Figure 4. LX Waveform with Damping Switch (with 200
Ω
external resistor)
8
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