Rainbow Electronics MAX1676 - Manual (Page 8)

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.
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.
Shutdown
The device enters shutdown when V SHDN is low
(V SHDN <20% of V OUT ). For normal operation, drive
SHDN high (V SHDN >80% of V
OUT
) or connect SHDN
to OUT. During shutdown, the body diode of the P-
channel MOSFET allows current flow from the battery to
the output. 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.
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.
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 = 200
Ω 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).
High-Efficiency, Low-Supply-Current,
Compact, Step-Up DC-DC Converters
8 _______________________________________________________________________________________
MAX1676
DAMPING
SWITCH
BATT
R1
200
Ω
LX
OUT
22 µH
V IN
0.1 µF 47 µF
V OUT
Figure 2. Simplified Diagram of Inductor Damping Switch
2 µs/div
V
LX
1V/div
Figure 3. LX Ringing Without Damping Switch
2 µs/div
V LX
1V/div
Figure 4. LX Waveform with Damping Switch (with 200 Ω
external resistor)