MAX1556_10 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – 16μA IQ, 1.2A PWM Step-Down DC-DC Converters

16µA I_Q, 1.2A PWM DC-DC
Step-Down Converters
Table 2. Inductor Selection
MANUFACTURER
PART
VALUE (µH)
Taiyo Yuden
LMNP04SB3R3N
3.3
Taiyo Yuden
LMNP04SB4R7N
4.7
TOKO
D52LC
3.5
TOKO
D52LC
4.7
Sumida
CDRH3D16
4.7
TOKO
D412F
4.7
Murata
LQH32CN
4.7
Sumitomo
CXL180
4.7
Sumitomo
CXLD140
4.7
*Estimated based upon similar-valued prototype inductors.
DCR (mΩ)
36
50
73
87
50
100*
97
70*
100*
I_SAT (mA)
1300
1200
1340
1140
1200
1200*
790
1000*
800*
SIZE (mm)
5 x 5 x 2.0
5 x 5 x 2.0
5 x 5 x 2.0
5 x 5 x 2.0
3.8 x 3.8 x 1.8
4.8 x 4.8 x 1.2
2.5 x 3.2 x 2.0
3.0 x 3.2 x 1.7
2.8 x 3.2 x 1.5
SHIELDED
Yes
Yes
Yes
Yes
Yes
Yes
No
No
No
V_DROPOUT = I_OUT x (R_DS(ON)P + DCR)
R_DS(ON)P is given in the Electrical Characteristics. DCR
for a few recommended inductors is listed in Table 2.
Soft-Start
The MAX1556/MAX1556A/MAX1557 use soft-start to
eliminate inrush current during startup, reducing tran-
sients at the input source. Soft-start is particularly use-
ful for higher-impedance input sources such as Li+ and
alkaline cells. Connect the required soft-start capacitor
from SS to GND. For most applications using a 22µF
output capacitor, connect a 1000pF capacitor from SS
to GND. If a larger output capacitor is used, then use
the following formula to find the value of the soft-start
capacitor:
C_SS
=
C_OUT
22000
Soft-start is implemented by exponentially ramping up
the output voltage from 0 to V_OUT(NOM) with a time con-
stant equal to C_SS times 200kΩ (see the Typical
Operating Characteristics). Assuming three time con-
stants to full output voltage, use the following formula to
calculate the soft-start time:
t_SS = 600 x 10³ x C_SS
Shutdown Mode
Connecting SHDN to GND or logic low places the
MAX1556/MAX1556A/MAX1557 in shutdown mode and
reduces supply current to 0.1µA. In shutdown, the con-
trol circuitry and the internal p-channel and n-channel
MOSFETs turn off and LX becomes high impedance.
Connect SHDN to IN or logic high for normal operation.
Thermal Shutdown
As soon as the junction temperature of the
MAX1556/MAX1556A/MAX1557 exceeds +160°C, the
ICs go into thermal shutdown. In this mode the internal
p-channel switch and the internal n-channel synchro-
nous rectifier are turned off. The device resumes nor-
mal operation when the junction temperature falls
below +145°C.
Applications Information
The MAX1556/MAX1556A/MAX1557 are optimized for
use with small external components. The correct selec-
tion of inductors and input and output capacitors
ensures high efficiency, low output ripple, and fast tran-
sient response.
Adjusting the Output Voltage
The MAX1556/MAX1556A/MAX1557 offer preset output
voltages of 1.0V, 1.2V, 1.3V, 1.5V, 1.8V, 2.5V, and 3.3V
as well as an adjustable output using external resistors.
Whenever possible, the preset outputs (set by D1 and
D2) should be used. With external resistor feedback,
noise coupling to FB can cause alternate LX pulse to
terminate early resulting in an inductor current wave-
form with alternate large and small current pulses. See
the External Feedback Switching Waveforms graph in
Typical Operating Characteristics section). Note that
external feedback and the alternating large-small pulse
waveform do not impact loop stability and have no
harmful effect on regulation or reliability.
The adjustable output is selected when D1 = D2 = 0
and an external resistor-divider is used to set the output
voltage (see Figure 6). The MAX1556/MAX1557 have a
defined line- and load-regulation slope. The load regu-
lation is for both preset and adjustable outputs and is
described in the Electrical Characteristics table and
Figures 4 and 5. The impact of the line-regulation slope
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