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参数资料
| 型号： | ISL8107EVAL2Z |
| 厂商： | Intersil |
| 文件页数： | 13/16页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD 2 FOR ISL8107 |
| 标准包装： | 1 |
| 系列： | * |
ISL8107
Rectifier Selection
Power Schottky diode is recommended for better converter
efficiency. The rectifier's rated reverse breakdown voltage
must be at least equal to the maximum input voltage,
preferably with a 20% derating factor. The power dissipation
is shown in Equation 11:
break frequency at F LC and a zero at F CE . The DC gain of
the modulator is simply the input voltage (V IN ) divided by the
peak-to-peak oscillator voltage Δ V OSC . The ISL8107
incorporates a feed forward loop that accounts for changes
in the input voltage. This maintains a constant modulator
gain.
P D [ W ] = I OUT ? V D ? ? 1 – ---------------- ?
? V OUT ?
? V IN ?
(EQ. 11)
For the purpose of this analysis, L and DCR represent the
output inductance and its DCR, while C and ESR represents
the total output capacitance and its equivalent series
where V D is the voltage of the Schottky diode = 0.5V to 0.7V
resistance in Equation 12.
F LC = ---------------------------
F CE = ---------------------------------
Application Guidelines
1
2 π ? L ? C
1
2 π ? C ? ESR
(EQ. 12)
Feedback Compensation
Figure 15 highlights the voltage-mode control loop for a buck
converter with type-III compensator. The output voltage is
regulated to the reference voltage level. The error amplifier
output is compared with the oscillator ramp wave to provide
a pulse-width modulated wave with an amplitude of V IN at
the PHASE node. The PWM wave is smoothed by the output
filter. The output filter capacitor bank’s equivalent series
resistance is represented by the series resistor ESR.
The compensation network consists of the transconductance
amplifier (internal to the ISL8107) and the external R 1 to R 4 ,
C 1 to C 3 components. The goal of the compensation network
is to provide a closed loop transfer function with high 0dB
crossing frequency (F 0 ; typically 0.1 to 0.3 of F SW ) and
adequate phase margin (better than 45°). Phase margin is the
difference between the closed loop phase at F 0dB and 180°.
The equations that follow relate the compensation network’s
poles, zeros and gain to the components (R 1 , R 2 , R 3 , R 4 , C 1 ,
-
-
C 1 = -------------------------------------
Δ V OSC
OSC
PWM
COMPARATOR
+
Z FB
V E/A
+
DRIVER
DRIVER
Z IN
V IN
L O
LX
C O
D
ESR
(PARASITIC)
V OUT
C 2 , and C 3 ) in Figures 4 and 5. Use the following guidelines
for locating the poles and zeros of the compensation network:
1. Select a value for R 2 , (10k to 100k typically)
2. Calculate C 1 such that F Z1 is placed at a fraction of the F LC ,
at 0.1 to 0.75 of F LC . The higher the quality factor of the
output filter and/or the higher the ratio F CE /F LC , the lower
the F Z1 frequency (to maximize phase boost at F LC ).
1
(EQ. 13)
2 π × F Z1 × R 2
ERROR
AMP
REFERENCE
3. Calculate C 3 such that F BW is placed at desired frequency
(typically, 0.1x to 0.5x F SW ). F SW represents the
DETAILED COMPENSATION COMPONENTS
switching frequency of the regulator.
V IN × R 2
C 2
Z FB
Z IN
V OUT
2 π × F BW × L × C 0 × V OSC
C 3 = ------------------------------------------------------------------------
(EQ. 14)
C 1
R 2
C 3
R 3
ISL8107 has feed forward compensation that adjusts the
amplitude of 0.11*V IN . Therefore, the Equation 14 can be
C 3 = ---------------------------------------------------------
COMP
g m -
+
FB
R 4
R 1
simplified as Equation 15:
0.22 π × F BW × L × C 0
R 2
(EQ. 15)
ISL8107
V REF
4. Calculate C 2 such that the placement of F P2 is at a fraction
of the F SW . The lowering of the frequency helps reduce
the gain of the compensation network at high frequency,
V OUT = V REF × ? 1 + ------ 1 - ?
C 2 = -------------------------------------
2 π × F P2 × R 2
? R ?
? R 4 ?
FIGURE 15. VOLTAGE-MODE BUCK CONVERTER
COMPENSATION DESIGN AND OUTPUT
VOLTAGE SELECTION
in turn reducing the HF ripple component at the COMP
pin and minimizing resultant duty cycle jitter.
1
(EQ. 16)
5. Calculate R 3 such that the placement of F P1 is at the F CE .
R 3 = --------------------------------------
The modulator transfer function is the small-signal transfer
function of V OUT /V COMP . This function is dominated by a
DC gain and shaped by the output filter, with a double pole
13
1
2 π × C 3 × F CE
(EQ. 17)
FN6605.0
October 29, 2008
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