ISL8104EVAL1Z Intersil, ISL8104EVAL1Z Datasheet - Page 10

• Manufacturer Part Number: ISL8104EVAL1Z
• Description: EVALUATION BOARD FOR ISL8104
• Manufacturer: Intersil

Specifications of ISL8104EVAL1Z

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 20A
• Voltage - Input: 8 ~ 14.4V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL8104
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

It is recommended that a mathematical model be used to
plot the loop response. Check the loop gain against the error
amplifier’s open-loop gain. Verify phase margin results and
adjust as necessary. Equation 13 describes the frequency
response of the modulator (G
(G
COMPENSATION BREAK FREQUENCY EQUATIONS
Figure 8 shows an asymptotic plot of the DC/DC converter’s
gain vs frequency. The actual Modulator Gain has a high gain
peak dependent on the quality factor (Q) of the output filter,
which is not shown. Using the previously mentioned guidelines
should yield a compensation gain similar to the curve plotted.
The open loop error amplifier gain bounds the compensation
gain. Check the compensation gain at F
capabilities of the error amplifier. The closed loop gain, G
constructed on the log-log graph of Figure 8 by adding the
modulator gain, G
compensation gain, G
multiplying the modulator transfer function and the
compensation transfer function and then plotting the
resulting gain.
A stable control loop has a gain crossing with close to a
-20dB/decade slope and a phase margin greater than 45°.
Include worst case component variations when determining
G
F
F
Z1
Z2
MOD
G
G
FB
FB
CL
times f
regulator. Change the numerical factor (0.7) below to
reflect desired placement of this pole. Placement of F
lower in frequency helps reduce the gain of the
compensation network at high frequency, in turn reducing
the HF ripple component at the COMP pin and minimizing
resultant duty cycle jitter.
R
C
=
=
) and closed-loop response (G
3
3
f ( )
f ( )
f ( )
------------------------------ -
2π R
-------------------------------------------------
2π
=
=
=
=
------------------- -
---------------------------------------------- -
2π R
=
f
---------- - 1
F
⋅
⋅
SW
SW
(
LC
1
R
D
------------------------------ -
--------------------------------------------------- - ⋅
s f ( ) R
------------------------------------------------------------------------------------------------------------------------ -
(
G
R
2
1
⋅
1
1
1
MAX
MOD
). f
⋅
–
+
V
+
1
+
C
3
OSC
⋅
s f ( ) R
SW
1
R
s f ( ) R
⋅
1
0.7 f
⋅
f ( ) G
3
MOD
1
V
) C
⋅
⋅
represents the switching frequency of the
⋅
IN
⋅
(
⋅
⋅
C
1
FB
3
SW
3
⋅
2
1
+
FB
(in dB), to the feedback
⋅
---------------------------------------------------------------------------------------------------------- -
1
⋅
+
C
s f ( )
(in dB). This is equivalent to
C
+
f ( )
C
3
1
s f ( )
)
2
F
F
⋅
)
⋅
10
P1
P2
MOD
⎛
⎜
⎝
(
1
R
⋅
(
1
+
=
=
ESR
+
where s f ( )
s f ( ) R
), feedback compensation
1
-------------------------------------------- -
2π R
------------------------------ -
2π R
R
CL
+
3
s f ( ) ESR C
⋅
⋅
⋅
+
) C
):
P2
⋅
DCR
1
,
2
2
3
against the
⋅
⋅
3
⋅
⋅
1
⎛
⎜
⎝
-------------------- -
C
C
C
-------------------- -
C
C
) C
3
1
1
=
1
⋅
1
+
⋅
+
⋅
C
2π f j
⋅
C
C
C
+
2
2
2
2
s
⋅ ⋅
⎞
⎟
⎠
2
⎞
⎟
⎠
f ( ) L C
(EQ. 12)
(EQ. 13)
(EQ. 14)
CL
⋅
, is
P2
⋅
ISL8104
phase margin. The mathematical model presented makes a
number of approximations and is generally not accurate at
frequencies approaching or exceeding half the switching
frequency. When designing compensation networks, select
target crossover frequencies in the range of 10% to 30% of
the switching frequency, f
Component Selection Guidelines
Output Capacitor Selection
An output capacitor is required to filter the output and supply
the load transient current. The filtering requirements are a
function of the switching frequency and the ripple current.
The load transient requirements are a function of the slew
rate (di/dt) and the magnitude of the transient load current.
These requirements are generally met with a mix of
capacitors and careful layout.
For applications that have transient load rates above 1A/ns,
high frequency capacitors initially supply the transient and
slow the current load rate seen by the bulk capacitors. The
bulk filter capacitor values are generally determined by the
ESR (effective series resistance) and voltage rating
requirements rather than actual capacitance requirements.
High frequency decoupling capacitors should be placed as
close to the power pins of the load as physically possible. Be
careful not to add inductance in the circuit board wiring that
could cancel the usefulness of these low inductance
components. Consult with the manufacturer of the load on
specific decoupling requirements.
Use only specialized low-ESR capacitors intended for
switching-regulator applications for the bulk capacitors.
The bulk capacitor’s ESR will determine the output ripple
voltage and the initial voltage drop after a high slew-rate
transient. An aluminum electrolytic capacitor's ESR value is
related to the case size with lower ESR available in larger
case sizes. However, the equivalent series inductance
(ESL) of these capacitors increases with case size and can
reduce the usefulness of the capacitor to high slew-rate
FIGURE 8. ASYMPTOTIC BODE PLOT OF CONVERTER GAIN
0
LOG
20
log
⎛
⎝
R2
------- -
R1
⎞
⎠
F
Z1
F
F
LC
Z2
F
F
SW
CE
P1
.
20
F
log
0
D
----------------------------------
F
MAX
P2
V
COMPENSATION GAIN
OSC
OPEN LOOP E/A GAIN
G
CLOSED LOOP GAIN
G
⋅
CL
MOD
MODULATOR GAIN
V
IN
FREQUENCY
March 7, 2008
FN9257.2
G
FB