參數(shù)資料
型號(hào)： MAX1994ETM
廠(chǎng)商： MAXIM INTEGRATED PRODUCTS INC
元件分類(lèi)： 穩(wěn)壓器
英文描述： Dual Step-Down Controllers Plus Linear- Regulator Controller for Notebook Computers
中文描述： DUAL SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, CQCC48
封裝： 7 X 7 MM, 0.80 MM HEIGHT, THIN, QFN-48
文件頁(yè)數(shù)： 30/49頁(yè)
文件大?。?/td> 983K
代理商： MAX1994ETM
M
Dual Step-Down Controllers Plus Linear-
Regulator Controller for Notebook Computers
30
______________________________________________________________________________________
Like BUCK1, the current-limit circuit of BUCK2 also
employs
“
valley
”
current sensing (Figure 6). If the magni-
tude of the current-sense voltage at CS2 is above the
current-limit threshold, the PWM is not allowed to initiate a
new cycle. The actual peak current is greater than the
current-limit threshold by an amount equal to the inductor
ripple current. Therefore, the exact current-limit character-
istic and maximum load capability are a function of the
sense resistance, inductor value, and battery voltage.
There is also a negative current limit that prevents
excessive reverse inductor currents when V
OUT2
is
sinking current in PWM mode. The negative current-
limit threshold is set to approximately 140% of the posi-
tive current limit and therefore tracks the positive
current limit when ILIM2 is adjusted.
The current-limit threshold is adjusted with an external
resistive voltage-divider at ILIM2. A 10μA (min) divider
current is recommended. The current-limit threshold
adjustment range is from 25mV to 250mV. In adjustable
mode, the current-limit threshold voltage is precisely
1/10th of the voltage at ILIM2. The threshold defaults to
50mV when ILIM2 is connected to V
CC
. The logic
threshold for switchover to the 50mV default value is
approximately V
CC
- 1V. The default current limit
accommodates the low voltage drop expected across
the sense resistor.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the cur-
rent-sense signal seen by CS2. Because CS2 is not a
real differential current-sense input, minimize the return
impedance from the sense resistor to the power ground
to reduce voltage errors when measuring the current.
In Figure 1, the Schottky diode (D2) provides a current
path parallel to the Q4/R2 current path. Accurate cur-
rent sensing demands D2 to be off while Q4 conducts.
Avoid large current-sense voltages. The combined volt-
age across Q4 and R2 can cause D2 to conduct. If very
large sense voltages are used, connect D2 directly
from Q4
’
s source to drain.
Forced-PWM Mode
BUCK1 and BUCK2 operate in forced-PWM mode
when SKP1/
SDN
and SKP2/
SDN
are unconnected. The
low-noise forced-PWM mode disables the zero-cross-
ing comparator, allowing the inductor current to reverse
at light loads. This causes the low-side gate-drive
waveform to become the complement of the high-side
gate-drive waveform. This in turn causes the inductor
current to reverse at light loads while DH_ maintains a
duty factor of V
OUT_
/V
IN
. The benefit of forced-PWM
mode is to keep the switching frequency fairly constant,
but it comes at a cost: the no-load 5V bias supply cur-
rent can be 20mA to 80mA total for both BUCK1 and
BUCK2, depending on the external MOSFETs and
switching frequency.
Forced-PWM mode is most useful for reducing audio-
frequency noise, improving load-transient response,
providing sink-current capability for dynamic-output
voltage adjustment, and improving the cross-regulation
of multiple-output applications that use a flyback trans-
former or coupled inductor. BUCK1 uses PWM mode
during all output transitions, while the slew-rate con-
troller is active and for 32 clock cycles thereafter.
Automatic Pulse-Skipping Mode
In skip mode (SKP_/
SDN
= high), an inherent automatic
switchover to PFM takes place at light loads. This
switchover is affected by a comparator that truncates
the low-side switch on-time at the inductor current
’
s
zero crossing. This mechanism causes the threshold
between pulse-skipping PFM and nonskipping PWM
operation to coincide with the boundary between con-
tinuous and discontinuous inductor-current operation
(also known as the
“
critical conduction
”
point).
In low duty-cycle applications, this threshold is relative-
ly constant, with only a minor dependence on battery
voltage.
where K is the on-time scale factor (Table 3). The load
current level at which PFM/PWM crossover occurs,
I
LOAD(SKIP)
, is equal to 1/2 the peak-to-peak ripple cur-
rent, which is a function of the inductor value (Figure 7).
For example, in the standard application circuit with K
= 3.3μs (Table 3), V
OUT1
= 1.25V, V
IN
= 12V, and L1 =
0.68μH, switchover to pulse-skipping operation occurs
at I
LOAD1
= 2.7A. The crossover point occurs at an
even lower value if a swinging (soft-saturation) inductor
is used.
The switching waveforms can appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency are made by varying the
inductor value. Generally, low inductor values produce
a broader efficiency vs. load curve, while higher values
result in higher full-load efficiency (assuming that the
coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values
include larger physical size and degraded load-tran-
sient response, especially at low-input-voltage levels.
I
KV
L
V
V
V
LOAD
SKIP
_(
OUT
2
IN
OUT
IN
)
_
_
_
=
×
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MAX1994ETM+ 功能描述:DC/DC 開(kāi)關(guān)控制器 Integrated Circuits (ICs) RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
MAX1994ETM+T 功能描述:DC/DC 開(kāi)關(guān)控制器 Integrated Circuits (ICs) RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
MAX1994ETM-T 功能描述:DC/DC 開(kāi)關(guān)控制器 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
MAX1994EVKIT 功能描述:DC/DC 開(kāi)關(guān)控制器 Evaluation Kit for the MAX1816 MAX1980 MAX1994 RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開(kāi)關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
MAX1995EGI 功能描述:顯示驅(qū)動(dòng)器和控制器 RoHS:否 制造商:Panasonic Electronic Components 工作電源電壓:2.7 V to 5.5 V 最大工作溫度: 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-44 封裝:Reel
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