MAX1556_1 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
(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/MAX1557 use soft-start to eliminate
inrush current during startup, reducing transients at the
input source. Soft-start is particularly useful 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:
thermal shutdown. In this mode the internal p-channel
switch and the internal n-channel synchronous rectifier
are turned off. The device resumes normal operation
when the junction temperature falls below +145°C.
Applications Information
The MAX1556/MAX1557 are optimized for use with small
external components. The correct selection of inductors
and input and output capacitors ensures high efficiency,
low output ripple, and fast transient response.
Adjusting the Output Voltage
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
can be reduced by applying a correction factor to the
feedback resistor equation.
First, calculate the correction factor, k, by plugging the
desired output voltage into the following formula:
t_SS = 600 x 10³ x C_SS
Shutdown Mode
Connecting SHDN to GND or logic low places the
MAX1556/MAX1557 in shutdown mode and reduces
supply current to 0.1µA. In shutdown, the control cir-
cuitry 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/MAX1557 exceeds +160°C, the ICs go into
k
=
1.06
x 10^−2V
x



V_OUTPUT −
3.6V
0.75V



k represents the shift in the operating point at the feed-
back node (OUT).
Select the lower feedback resistor, R3, to be ≤35.7kΩ to
ensure stability and solve for R2:
 0.75V − k 
 V_OUTPUT 
=
R3
(^{R3 +R2})
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