Rainbow Electronics MAX1724 - Manual (Page 8)

MAX1722/MAX1723/MAX1724
1.5µA I Q , Step-Up DC-DC Converters
in Thin SOT23-5
8 _______________________________________________________________________________________
BATT/Damping Switch
(MAX1722/MAX1724)
The MAX1722/MAX1724 include an internal damping
switch (Figure 4) to minimize ringing at LX and reduce
EMI. 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 damping switch supplies a path to quickly
dissipate this energy, suppressing the ringing at LX.
This does not reduce the output ripple, but does
reduce EMI with minimal impact on efficiency. Figures
5 and 6 show the LX node voltage waveform without
and with the damping switch, respectively.
Design Procedure
Setting the Output Voltage
(MAX1722/MAX1723)
The output voltage can be adjusted from 2V to 5.5V
using external resistors R1 and R2 (Figure 7). Since FB
leakage is 20nA (max), select feedback resistor R1 in
the 100k
Ω to 1MΩ range. Calculate R2 as follows:
where V FB = 1.235V.
R R
V
V
OUT
FB
2 1 1
= −

 

MAX1722
MAX1724
PDRV
DAMP
NDRV
TIMING
CIRCUIT
OUT
V OUT
V IN
BATT
LX
DAMPING
SWITCH
GND
P
N
1 µs/div
1V/div
1 µs/div
1V/div
Figure 6. LX Ringing With Damping Switch (MAX1722/MAX1724) Figure 5. LX Ringing Without Damping Switch (MAX1723)
Figure 4. Simplified Diagram of Damping Switch