MAX15108 Datasheet, PDF (13/19 Pages) Maxim Integrated Products – High-Efficiency, 8A, Current-Mode Synchronous Step-Down Switching Regulator

High-Efficiency, 8A, Current-Mode
Synchronous Step-Down Switching Regulator
Make sure that the selected capacitance can accom-
modate the input ripple current given by:
I_RMS = I_O ×
V_OUT × (V_IN - V_OUT )
V_IN
If necessary, use multiple capacitors in parallel to meet
the RMS current rating requirement.
Output Capacitor Selection
Use low-ESR ceramic capacitors to minimize the voltage
ripple due to ESR. Use the following formula to estimate
the total output voltage peak-to-peak ripple:
∆V_OUT
=
V_OUT
f_SW × L

× _1-

V_OUT
V_IN



×

^R_{ESR_COUT}

+
8
×
1
f_SW ×
C_OUT



Select the output capacitors to produce an output ripple
voltage that is less than 2% of the set output voltage.
Setting the Soft-Start Startup Time
The soft-start feature ramps up the output voltage slowly,
reducing input inrush current during startup. Size the
CSS capacitor to achieve the desired soft-start time, tSS,
using:
C _SS
=
I_SS x t _SS
V_FB
VREF_EXT
RSS
CSS
SS
MAX15108
Figure 2. Setting Soft-Start Time
ISS, the soft-start current, is 10FA, and VFB, the output
feedback voltage threshold, is 0.6V. When using large
COUT capacitance values, the high-side current limit can
trigger during the soft-start period. To ensure the correct
soft-start time, tSS, choose CSS large enough to satisfy:
C _SS
>>
C_OUT
×
V_OUT × I_SS
(I_{HSCL_MIN} - I_OUT) ×
V_FB
IHSCL_MIN is the minimum high-side switch current-limit
value.
An external tracking reference with steady-state value
between 0V and VIN - 1.5V can be applied to SS. In this
case, connect an RC network from external tracking ref-
erence and SS as in Figure 2. Set RSS to approximately
1kI. In this application, RSS is needed to ensure that,
during hiccup period, SS can be internally pulled down.
When an external reference is connected to SS, the soft-
start must be provided externally.
Skip Mode Frequency and Output Ripple
In skip mode, the switching frequency (fSKIP) and output
ripple voltage (VOUT-RIPPLE) shown in Figure 3 are cal-
culated as follows:
tON is a fixed time by design (330ns, typ); the peak
inductor current reached is:
I_{SKIP−LIMIT}
=
V_IN − V_OUT
2× L
×
t _ON
tOFF1 is the time needed for the inductor current to reach
the zero-crossing (~0A):
t
OFF1
=
L
×
I_SKIP-LIMIT
V_OUT
IL
ISKIP-LIMIT
VOUT
tON tOFF1
t_OFF2 = n x t_CK
ILOAD
VOUT-RIPPLE
Figure 3. Skip-Mode Waveforms
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