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参数资料
| 型号： | MAX8795A+ |
| 厂商： | MAXIM INTEGRATED PRODUCTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | 1.6 A SWITCHING REGULATOR, 1400 kHz SWITCHING FREQ-MAX, QCC32 |
| 封装： | 5 X 5 MM, 0.80 MM HEIGHT, LEAD FREE, MO-220WHHD-2, TQFN-32 |
| 文件页数： | 13/24页 |
| 文件大小： | 387K |
| 代理商： | MAX8795A+ |
MAX8795A
TFT-LCD DC-DC Converter with
Operational Amplifiers
2
20
0
_
__
_
where VT is 26mV at room temperature, and IBIAS is the
current through the base-to-emitter resistor (RBE). For
the MAX8795A, the bias currents for both the gate-on
and gate-off linear-regulator controllers are 0.1mA.
Therefore, the base-to-emitter resistor for both linear
regulators should be chosen to set 0.1mA bias current:
The output capacitor and the load resistance create the
dominant pole in the system. However, the internal
amplifier delay, pass transistor’s input capacitance,
and the stray capacitance at the feedback node create
additional poles in the system, and the output capaci-
tor’s ESR generates a zero. For proper operation, use
the following equations to verify the linear regulator is
properly compensated:
1) First, determine the dominant pole set by the linear
regulator’s output capacitor and the load resistor:
The unity-gain crossover of the linear regulator is:
fCROSSOVER = AV_LR fPOLE_LR
2) The pole created by the internal amplifier delay is
approximately 1MHz:
fPOLE_AMP = 1MHz
3) Next, calculate the pole set by the transistor’s input
capacitance, the transistor’s input resistance, and
the base-to-emitter pullup resistor:
gm is the transconductance of the pass transistor, and fT
is the transition frequency. Both parameters can be found
in the transistor’s data sheet. Because RBE is much
greater than RIN, the above equation can be simplified:
Substituting for CIN and RIN yields:
4) Next, calculate the pole set by the linear regulator’s
feedback resistance and the capacitance between
FB_ and AGND (including stray capacitance):
where CFB is the capacitance between FB_ and
AGND, RUPPER is the upper resistor of the linear regu-
lator’s feedback divider, and RLOWER is the lower resis-
tor of the divider.
5) Next, calculate the zero caused by the output
capacitor’s ESR:
where RESR is the equivalent series resistance of
COUT_LR.
To ensure stability, choose COUT_LR large enough so
the crossover occurs well before the poles and zero
calculated in steps 2 to 5. The poles in steps 3 and 4
generally occur at several megahertz, and using
ceramic capacitors ensures the ESR zero occurs at
several megahertz as well. Placing the crossover below
500kHz is sufficient to avoid the amplifier-delay pole
and generally works well, unless unusual component
choices or extra capacitances move one of the other
poles or the zero below 1MHz.
Applications Information
Power Dissipation
An IC’s maximum power dissipation depends on the
thermal resistance from the die to the ambient environ-
ment and the ambient temperature. The thermal resis-
tance depends on the IC package, PCB copper area,
other thermal mass, and airflow.
The MAX8795A, with its exposed backside paddle sol-
dered to 1in2 of PCB copper and a large internal ground
plane layer, can dissipate approximately 2.76W into
+70
°C still air. More PCB copper, cooler ambient air,
and more airflow increase the possible dissipation, while
less copper or warmer air decreases the IC’s dissipation
capability. The major components of power dissipation
are the power dissipated in the step-up regulator and
the power dissipated by the operational amplifiers.
f
CR
POLE ESR
OUT LR
ESR
_
=
××
1
2
π
f
CR
R
POLE FB
FB
UPPER
LOWER
_
(||
)
=
××
1
2
π
f
h
POLE IN
T
FE
_
=
f
CR
POLE IN
IN
_
=
××
1
2
π
where
C
g
f
R
h
g
IN
m
T
IN
FE
m
:
,
==
2
π
f
CR
R
POLE IN
IN
BE
IN
_
(||
)
=
××
1
2
π
f
I
CV
POLE LR
LOAD MAX
LR
OUT LR
_
() _
__
=
××
2
π
R
V
mA
V
mA
k
BE
==≈
01
07
01
68
.
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