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参数资料
| 型号： | FAN5242QSCX |
| 文件页数： | 9/20页 |
| 文件大小： | 123K |
| 代理商： | FAN5242QSCX |
FAN5201
9
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When the current-sense amplifier is in control of the PWM,
the comparator adjusts the duty cycle of the switches, regu-
lating the average battery current and keeping it proportional
to the error voltage. The current is averaged, rather than
peak, since the current sense resistor is between the output
capacitor and the battery. Since the average battery current is
nearly the same as the peak current, the controller acts as a
transconductance amplifier, reducing the effect of the inductor
on the output filter LC formed by the output inductor and the
output capacitance. This makes stabilizing the circuit easy, since
the output filter changes from a complex second-order RLC
to a first-order RC. To preserve the inner current-control
loop’s stability, slope compensation is also fed into the com-
parator. This damps out perturbations in the pulse width at
duty ratios greater than 50%. At heavy loads, the PWM con-
troller switches at a fixed frequency and modulates the duty
cycle to control the battery current. At light loads, the DC
current through the inductor is not sufficient to prevent the
current from going negative through the synchronous rectifier
(Figure 2, Q4). The controller monitors the current through
the sense resistor; when it drops to below 200mA, the syn-
chronous rectifier turns off to prevent negative current flow.
When the voltage error amplifier is in control of the PWM,
the comparator adjusts the duty cycle of the switches, regu-
lating the battery voltage and keeping it proportional to the
error voltage. In this mode, the control loop is a standard
voltage-mode control, and the only requirement to guarantee
stability is that the loop gain be rolled off below 0dB before
the LC resonant frequency.
When the power error amplifier is in control of the PWM,
the comparator adjusts the duty cycle of the switches, regu-
lating the total power drawn from the charger. The loop
determines whether the total power available from the wall
adapter is sufficient to provide both the load and battery
charging needs. If not, the charging power to the battery is
reduced by the amount needed to keep the total demand
within the AC-DC output power limit of the adapter.
The PWM controller also implements voltage feedforward.
This means that the gain of the control loops are adjusted
inversely proportionally to the input voltage: as the input
voltage increases, loop gain is decreased. This improves the
audio susceptibility of the converter, and in particular, means
that the bandwidth of each of the loops is relatively indepen-
dent of the AC adapter voltage. Feedforward is accomplished
by modulating the amplitude of the ramp signal.
MOSFET Drivers
The FAN5201 drives external MOSFETs to regulate battery
voltage or current, a high-side P-channel and a low-side
N-channel for synchronous rectification. Use of a P-channel
MOSFET for the high-side switch permits operation without
charge-pumping and its attendant external components. The
synchronous rectifier behaves like a diode, but with a smaller
voltage drop to improve efficiency. A small dead time is
added between the time that the high-side MOSFET turns off
and the synchronous rectifier turns on, and vice versa. This
prevents shootthrough currents (currents that flow through
both MOSFETs during the brief time that one is turning on
and the other is turning off). A schottky rectifier from the
source to the drain of Q4 prevents the synchronous rectifier’s
body diode from conducting. The body diode typically has
slower switching-recovery times, so allowing it to conduct
would degrade efficiency.
Control and Reference Functions
The FAN5201 has a number of additional analog functions
to enhance overall system performance. The ACAV is an
open collector signal that can be used to determine the pres-
ence of the AC charger; its threshold is set by an external
resistor divider attached the Vth pin.
A 5V keep alive linear regulator receives power either from
the AC adapter via Q1 or from the battery (PSIN pin). This
regulator can provide up to 10mA to power memory during
a system shutdown.
Protection Circuitry
The FAN5201 protects against a variety of possible fault or
problem conditions.
Input Protection
Inrush current can be a problem during hot plug-in if in front
of the switching regulator a large capacitor is used to decouple
noise. Conceivably, the inrush could be high enough to trip
on overcurrent protection in the AC adapter. The FAN5201
provides the means for limiting inrush current to any desired
value: The SSIN/ILIM pin provides a sink current of 65μA
maximum to turn on the gate of the P-channel MOSFET Q1,
so that selecting a gate-source capacitance on Q1 will slow its
turn-on time to any desired speed, thus restricting the amount
of inrush current.
The charger has its own local soft start, which controls the
maximum duty cycle of the PWM. The softstart time is set
by selecting a capacitor attached to the SS pin. The softstart
pin can also be used for a hard shutdown, by pulling it to
ground.
While the AC adapter is not present, the FAN5201 shuts
itself off, using an UVLO set at 6.0V.
If the AC adapter is connected to the FAN5201 circuit but is
not plugged in, the adapter could present a load to the bat-
tery. The FAN5201 prevents this by turning off Q2 (attached
to the INISO pin) if the input current falls below 200mA
(with an 18m
sense resistor). However, this function is dis-
abled until the softstart pin reaches steady state.
Output Protection
If input current exceeds the design of the FAN5201 (6A with
an 18m
sense resistor) the IC latches off Q1, disconnecting
the circuitry from input power within a few microseconds.
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| FAN5250QSC | 功能描述:DC/DC 开关控制器 DC/DC RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
| FAN5250QSC_Q | 功能描述:DC/DC 开关控制器 DC/DC RoHS:否 制造商:Texas Instruments 输入电压:6 V to 100 V 开关频率: 输出电压:1.215 V to 80 V 输出电流:3.5 A 输出端数量:1 最大工作温度:+ 125 C 安装风格: 封装 / 箱体:CPAK |
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