Rainbow Electronics MAX15058 - Manual (Page 18)

High-Efficiency, 3A, Current-Mode
Synchronous, Step-Down Switching Regulator
MAX15058
18 _____________________________________________________________________________________
( )
( )
LOAD S
SW
C CO OUT
MV MC LOAD
S
LOAD SW
R K 1 D 0.5
1
L f
R1 R2
R 2 f C
R2 g g R
1
ESR
K 1 D 0.5 1
R L f
    − −
 
+  
  Ч
+  
= Ч Ч π Ч
Ч Ч
 
 
 
+  
    − −    
+  
 
  Ч
     
and where the ESR is much smaller than the parallel
combination of the equivalent load resistance and the
current loop impedance, e.g.,:
( )
S
LOAD SW
1
ESR
K 1 D 0.5 1
R L f
<<
    − −
 
+  
  ×
 
R C becomes:
CO OUT
C
MV MC
2 f C R1 R2
R
R2 g g
π Ч +
= Ч
Ч
3) Determine C
C
by selecting the desired first sys-
tem zero, f Z1 , based on the desired phase margin.
Typically, setting f Z1 below 1/5 of f CO provides suf-
ficient phase margin.
CO
Z1
C C
f 1
f
2 C R 5
= ≤
π ×
therefore:
C
CO C
5
C
2 f R
≥
π Ч Ч
4) For low duty-cycle applications, the addition of a
phase-leading capacitor (C FF in Figure 1) helps
mitigate 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:
( ) FF
CO
1
C
2 f R1|| R2
=
π Ч Ч
This guarantees the additional phase-leading zero
occurs at a frequency lower than f CO from:
PHASE_LEAD
FF
1
f
2 C R1
=
π Ч Ч
Using C FF the zero-pole order is adjusted as follows:
P1 P2 Z1
FF FF
CO P3 Z2
1 1
f f f
2 C R1 2 C (R1|| R2)
f f f
< ≤ < < ≈
π π
< <
Confirm the desired operation of C FF empirically. The
phase lead of C FF diminishes as the output voltage
is a smaller multiple of the reference voltage, e.g.,
below about 1V. Do not use C
FF when V OUT = V FB .
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:
SS SS
SS
FB
I t
C
V
×
=
I SS , the soft-start current, is 10FA (typ) and V FB , the
output feedback voltage threshold, is 0.6V (typ). 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, t
SS , choose C SS large
enough to satisfy:
OUT SS
SS OUT
HSCL_ OUT FB
V I
C C
(I I ) V
×
>> Ч
− Ч
I HSCL_ is the typical high-side MOSFET current-limit
value.
An exte
rnal tracking reference with steady-state value
between 0V and V IN - 1.8V can be applied to SS/REFIN.
In this case, connect an RC network from external track-
ing reference and SS/REFIN, as shown in Figure 4. The
recomm ended value for R SS is a pproximately 1kI. R SS
is needed to ensure that, during hiccup period, SS/
REFIN can be internally pulled down.
When an external reference is connected to SS/REFIN,
the soft-start must be provided externally.
Figure 4. RC Network for External Reference at SS/REFIN
C SS
R SS
V REF_EXT SS/REFIN
MAX15058