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| 型号： | MAX1531ETJ+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 18/33页 |
| 文件大小： | 0K |
| 描述： | IC PS CTRLR MULTI-OUTPUT 32TQFN |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | 五路电源监控器 |
| 电源电压： | 4.5 V ~ 28 V |
| 电流 - 电源： | 1.7mA |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 32-WFQFN 裸露焊盘 |
| 供应商设备封装： | 32-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
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Multiple-Output Power-Supply
Controllers for LCD Monitors
During the second half of the cycle, the high-side MOS-
SLOPE
SS DONE
∑
CURRENT
SENSE
AND
CURRENT
LIMIT
IN
FET turns off and the low-side N-channel MOSFET turns
on. Now the inductor releases the stored energy as its
current ramps down, providing current to the output.
The output capacitor stores charge when the inductor
DC-DC EN
current exceeds the load current and discharges when
V REF
FB
COMP
CLOCK
SOFT-
START
GM
PWM COMP
R
S
Q
Q
DH
DL
the inductor current is lower, smoothing the voltage
across the load. Under overload conditions, when the
inductor current exceeds the selected current limit (see
Current Limit Circuit ), the high-side MOSFET is not
turned on at the rising edge of the clock and the low-
side MOSFET remains on to let the inductor current
ILIM
CURRENT
LIMIT
LX
PGND
ramp down.
Under light-load conditions, the MAX1530/MAX1531
maintain a constant switching frequency to minimize
0.9VREF
FAULT COMPARATOR
FLTM
cross-regulation errors in applications that use a trans-
former. The low-side gate-drive waveform is the comple-
ment of the high-side gate-drive waveform, which
Figure 4. Step-Down Controller Block Diagram
pumps attached to the switching node or extra wind-
ings coupled to the step-down converter inductor. The
negative gain block (MAX1531) can be used in con-
junction with a charge pump or coupled winding to
generate the LCD gate-off voltage or other negative
supplies.
Step-Down Controller
The MAX1530/MAX1531 include step-down controllers
that use a fixed-frequency current-mode PWM control
scheme (Figure 4). An internal transconductance
amplifier establishes an integrated error voltage at the
COMP pin. The heart of the current-mode PWM con-
troller is an open-loop comparator that compares an
integrated voltage-feedback signal with an amplified
current-sense signal plus a slope-compensation ramp.
At each rising edge of the internal clock, the high-side
MOSFET turns on until the PWM comparator trips or the
maximum duty cycle is reached. During this on-time,
current ramps up through the inductor, sourcing cur-
rent to the output and storing energy in a magnetic
field. The current-mode feedback system regulates the
peak inductor current as a function of the output volt-
age error signal. Since the average inductor current is
nearly the same as the peak inductor current (assum-
ing that the inductor value is relatively high to minimize
ripple current), the circuit acts as a switch-mode
transconductance amplifier. That pushes the output LC
filter pole, normally found in a voltage-mode PWM, to a
higher frequency. To preserve loop stability, the slope-
compensation ramp is summed into the main PWM
comparator.
causes the inductor current to reverse under light loads.
Current-Sense Amplifier
The MAX1530/MAX1531s’ current-sense circuit ampli-
fies the current-sense voltage generated by the high-
side MOSFET ’s on-resistance. This amplified
current-sense signal and the internal slope compensa-
tion signal are summed together and fed into the PWM
comparator’s inverting input. Place the high-side MOS-
FET near the controller, and connect IN and LX to the
MOSFET using Kelvin-sense connections to guarantee
current-sense accuracy and improve stability.
Current-Limit Circuit
The MAX1530/MAX1531 include two current-limit cir-
cuits that use the two MOSFETs’ on-resistances as cur-
rent-sensing elements (Figure 4). The high-side
MOSFET’s voltage is used with a fixed 400mV (typ) cur-
rent-limit threshold during the high-side on-times. The
low-side MOSFET’s voltage is used with an adjustable
current-limit threshold during the low-side on-times.
Using both circuits together ensures that the current is
always measured and controlled.
The high-side MOSFET current limit employs a peak
current limit. If the voltage across the high-side MOS-
FET, measured from IN to LX, exceeds the 400mV
threshold during an on-time, the high-side MOSFET
turns off and the low-side MOSFET turns on.
The low-side MOSFET current-limit circuit employs a
“valley” current limit. If the voltage across the low-side
MOSFET, measured from LX to PGND, exceeds the
low-side threshold at the end of a low-side on-time, the
low-side MOSFET remains on and the high-side MOS-
FET stays off for the entire next cycle.
18
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