Datasheet

MAX1858
• R
COMP
has much higher impedance than C
COMP_A
.
This is true if, and only if, crossover occurs above
f
Z_COMP_A
. If this is true, C
COMP_A
can be ignored
(as a short to ground).
• R
ESR
is much higher impedance than C
OUT
. This is
true if, and only if, crossover occurs well after the out-
put capacitor’s ESR zero. If this is true, C
OUT
becomes an insignificant part of the loop gain and can
be ignored (as a short to ground).
• C
COMP_B
is much higher impedance than R
COMP
and can be ignored (as an open circuit). This is true
if, and only if, crossover occurs far below f
COMP_B
.
The following loop-gain equation can be found by using
these previous approximations with Figure 7:
Setting the loop gain to 1 and solving for the crossover
frequency yields:
To ensure stability, select R
COMP
to meet the following
criteria:
• Unity-gain crossover must occur below 1/5th of the
switching frequency.
• For reasonable phase margin using type 1 compen-
sation, f
CO
must be larger than 5 ✕ f
ESR
.
Choose C
COMP_A
so that f
Z_COMP_A
equals half f
LC
using the following equation:
Choose C
COMP_B
so that f
COMP_B
occurs at 3 times
f
CO
using the following equation:
MOSFET Selection
The MAX1858’s step-down controller drives two exter-
nal logic-level N-channel MOSFETs as the circuit switch
elements. The key selection parameters are:
• On-resistance (R
DS(ON)
)
• Maximum drain-to-source voltage (V
DS(MAX)
)
• Minimum threshold voltage (V
TH(MIN)
)
• Total gate charge (Q
g
)
• Reverse transfer capacitance (C
RSS
)
• Power dissipation
C
fR
COMP B
CO COMP
_
=
××
()
×
1
23π
C
LC
R
COMP A
OUT
COMP
_
=
×2
f GBW
V
V
V
V
gRR
L
CO
IN
RAMP
SET
OUT
M COMP COMP ESR
== ×
×
××
×
_
2π
A
V
V
V
V
gRR
sL
L
IN
RAMP
SET
OUT
M COMP COMP ESR
≅××
××
_
Dual 180° Out-of-Phase PWM Step-Down
Controller with Power Sequencing and POR
16 ______________________________________________________________________________________
COMP_
R
COMP_
C
COMP_A
C
COMP_B
g
M_GROUP
P
W
M
V
C
V
SET
DH
DL
N
N
L
LX
FB
V
OUT
R
ESR
C
OUT
COMP_
R
COMP_
C
COMP_A
C
COMP_B
g
M_GROUP
V
SET
L
LX
FB
R
ESR
C
OUT
GAIN = +V
IN
/V
RAMP
FOR
FREQUENCIES LOWER
THAN NYQUIST
=
Figure 7. Fixed-Frequency Voltage-Mode Control Loop