MAX15118 Datasheet, PDF (20/23 Pages) Maxim Integrated Products – High-Efficiency, 18A, Current-Mode Synchronous Step-Down Regulator with Integrated Switches

MAX15118
High-Efficiency, 18A, Current-Mode Synchronous
Step-Down Regulator with Integrated Switches
3) Select C_C. C_C is determined by selecting the desired
first system zero, f_Z1, based on the desired phase
margin. Typically, setting f_Z1 below 1/5th of f_CO pro-
vides sufficient phase margin.
C_C
≥
2π
5
f_CO
×
R_C
Optionally, for low duty-cycle applications, the addition
of a phase-leading capacitor (C_FF in Figure 2) helps miti-
gate the phase lag of the damped half-frequency double
pole. Adding a second zero near to but below the desired
crossover frequency increases both the closed-loop
phase margin and the regulator’s unity-gain bandwidth
(crossover frequency). Select the capacitor as follows:
C_FF
=
2π
×
f_CO
1
× (R1
||
R2)
Using C_FF, the zero-pole order is adjusted as follows:
f_P1 < f_P2 < f_Z1 < 1/ [2πC_FFR₁]
< 1/ _2πC_FF (R1|| R2)_ < f_P3 < f_Z2
Setting the Soft-Start Time
The soft-start feature ramps up the output voltage slowly,
reducing input inrush current during startup. Size the C_SS
capacitor to achieve the desired soft-start time, t_SS, using:
C _SS
=
I_SS × t _SS
V_FB
ISS, the soft-start current, is 10FA (typ) and V_FB is the
0.6V (typ) output feedback voltage threshold. When
using large C_OUT capacitance values, the high-side
current limit can trigger during the soft-start period. To
ensure the correct soft-start time, tSS, choose C_SS large
enough to satisfy:
C _SS
>>
C
OUT
×
V_OUT × I_SS
(24A - I_LOAD) ×
V_FB
An external tracking reference with steady-state value
between 0V and (V_IN - 2.5V) can be applied to SS/REFIN.
In this case, connect an RC network from the external
track­ing reference and SS/REFIN, as shown in Figure 4.
The recommended value for RSS is approximately 330I.
R_SS is needed to ensure that, during hiccup period,
SS/REFIN can be pulled down internally.
VREF_EXT
RSS
CSS
SS/REFIN
MAX15118
Figure 4. RC Network for External Reference at SS/REFIN
Design Examples
Table 1 provides values for various outputs based on the
typical operating circuit.
Table 1. Suggested Component Values (see the Typical Operating Circuits)
V_IN (V)
3.3
V_OUT (V)
0.8
L (μH)
0.15
LIR (A/A)
0.22
C15 (nF)
6.8
R3 (kI)
2.94
C14 (pF)
22
3.3
1.2
0.15
0.28
4.7
2.21
22
3.3
1.5
0.15
0.30
3.3
3.83
22
3.3
1.8
0.15
0.30
3.3
4.22
22
3.3
2.5
0.15
0.22
3.3
5.62
22
5
0.8
0.15
0.25
6.8
2.94
22
5
1.2
0.15
0.34
4.7
2.21
22
5
1.5
0.15
0.39
3.3
3.83
22
5
1.8
0.22
0.29
3.3
3.92
22
5
2.5
0.22
0.32
3.3
5.1
22
5
3.3
0.22
0.28
2.2
4.64
22
Note: C_IN, C_OUT, and other components are the same as in the standard MAX15118 Evaluation Kit.
R1 (kI)
1.78
5.36
8.06
10.7
16.9
1.78
5.36
8.06
10.7
16.9
24.3
R2 (kI)
5.36
5.36
5.36
5.36
5.36
5.36
5.36
5.36
5.36
5.36
5.36
���������������������������������������������������������������� Maxim Integrated Products   20