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| 型号： | MAX8739ETP+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 15/19页 |
| 文件大小： | 0K |
| 描述： | IC DC/DC CONV W/OPAMP 20TQFN-EP |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | 转换器，TFT，LCD |
| 输入电压： | 1.8 V ~ 5.5 V |
| 输出数： | 1 |
| 输出电压： | 1.8 V ~ 13 V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-WQFN 裸露焊盘 |
| 供应商设备封装： | 20-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
TFT, LCD, DC-DC Converter with
Operational Amplifiers
Undervoltage Lockout (UVLO)
The undervoltage lockout (UVLO) circuit compares the
input voltage at IN with the UVLO threshold (1.26V ris-
ing and 1.1V falling) to ensure that the input voltage is
high enough for reliable operation. The 200mV (typ)
hysteresis prevents supply transients from causing a
restart. Once the input voltage exceeds the UVLO ris-
ing threshold, startup begins. When the input voltage
falls below the UVLO falling threshold, the controller
turns off the main step-up regulator and the linear regu-
lator outputs, disables the switch-control block, and the
operational amplifier outputs are high impedance.
Linear Regulator (LDO)
The MAX8739 includes an internal 5V linear regulator.
SUP is the input of the linear regulator. The input voltage
range is between 4.5V and 13V. The output of the linear
regulator (LDO) is set to 5V (typ). The regulator powers
all the internal circuitry including the gate driver. Bypass
the LDO pin to AGND with a 0.22μF or greater ceramic
capacitor. SUP should be directly connected to the out-
put of the step-up regulator. This feature significantly
improves the efficiency at low-input voltages.
Bootstrapping and Soft-Start
The MAX8739 features bootstrapping operation. In nor-
mal operation, the internal linear regulator supplies
power to the internal circuitry. The input of the linear
regulator (IN) should be directly connected to the out-
put of the step-up regulator. The MAX8739 is enabled
when the input voltage at SUP is above 1.3V (typ) and
the fault latch is not set. After being enabled, the regu-
lator starts open-loop switching to generate the supply
voltage for the linear regulator. The internal reference
block turns on when the LDO voltage exceeds 2.7V
(typ). When the reference voltage reaches regulation,
the PWM controller and the current-limit circuit are
enabled, and the step-up regulator enters soft-start.
During soft-start, the main step-up regulator directly
limits the peak-inductor current, allowing from zero up
to the full current-limit value in eight equal current steps
(ILIM/8). The maximum load current is available after
the output voltage reaches regulation (which terminates
soft-start), or after the soft-start timer expires in approxi-
mately 13ms. The soft-start routine minimizes the inrush
current and voltage overshoot and ensures a well-
defined startup behavior.
Fault Protection
During steady-state operation, the MAX8739 monitors
the FB voltage. If the FB voltage does not exceed 1V
(typ), the MAX8739 activates an internal fault timer. If
there is a continuous fault for the fault-timer duration,
the MAX8739 sets the fault latch, shutting down all the
outputs. Once the fault condition is removed, cycle the
input voltage to clear the fault latch and reactivate the
device. The fault-detection circuit is disabled during the
soft-start time.
The MAX8739 monitors the SUP voltage for undervolt-
age and overvoltage conditions. If the SUP voltage is
below 1.4V (max) or above 13.7V (typ), the MAX8739
disables the gate driver of the step-up regulator and
prevents the internal MOSFET from switching. The SUP
undervoltage and overvoltage conditions do not set the
fault latch.
Thermal-Overload Protection
The thermal-overload protection prevents excessive
power dissipation from overheating the device. When
the junction temperature exceeds T J = +160°C, a ther-
mal sensor immediately activates the fault protection,
which shuts down the step-up regulator and the internal
linear regulator, allowing the device to cool down. Once
the device cools down by approximately 15°C, cycle
the input voltage (below the UVLO falling threshold) to
clear the fault latch and reactivate the device.
The thermal-overload protection protects the controller
in the event of fault conditions. For continuous opera-
tion, do not exceed the absolute maximum junction
temperature rating of T J = +150°C.
Design Procedure
Main Step-Up Regulator
Inductor Selection
The minimum inductance value, peak-current rating,
and series resistance are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output-load capability,
transient response time, and output-voltage ripple.
Physical size and cost are also important factors to be
considered.
The maximum output current, input voltage, output volt-
age, and switching frequency determine the inductor
value. Very-high inductance values minimize the cur-
rent ripple and therefore reduce the peak current,
which decreases core losses in the inductor and I 2 R
losses in the entire power path. However, large induc-
tor values also require more energy storage and more
turns of wire, which increase physical size and can
increase I 2 R losses in the inductor. Low-inductance val-
ues decrease the physical size but increase the current
ripple and peak current. Finding the best inductor
involves choosing the best compromise between circuit
efficiency, inductor size, and cost.
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