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| 型号： | MAX1897ETP+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 16/33页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM 20-TQFN |
| 标准包装： | 2,500 |
| 系列： | Quick-PWM™ |
| PWM 型： | 控制器 |
| 输出数： | 1 |
| 频率 - 最大： | 550kHz |
| 电源电压： | 4.5 V ~ 5.5 V |
| 降压： | 是 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | 0°C ~ 85°C |
| 封装/外壳： | 20-WQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
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Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
Table 4. Operating Mode Truth Table
SHDN
GND
ILIM
X
DL
High
MODE
Shutdown
COMMENTS
Micropower, shutdown mode (I CC +I DD < 1μA typ). DL forced high,
DH forced low, and the PWM controller disabled.
Low-power, standby mode (I CC + I DD = 20μA typ). DL forced high,
V CC
GND
(< 0.25V)
High
Standby
DH forced low, and the PWM controller disabled. However, the bias
and fault protection circuitry remain active so the MAX1887/MAX1897
can continuously monitor the ILIM input.
V CC
High
(> 0.25V)
Switching
Normal
Operation
Low-noise, fixed-frequency, PWM operation. The inductor current
reverses with light loads.
X = Don ’ t Care
Several Quick-PWM converters that may be used as the
master controller ramp down the output voltage at a
controlled slew rate when shut down. When combined
with these master controllers, the MAX1897 must not be
deactivated until the output voltage is fully discharged.
Otherwise the slave ’ s low side switch will turn on while
the master is still attempting to regulate the output. In
these applications, delay the shutdown input signal to
the MAX1897 or permanently connect SHDN to V CC
and use standby mode to conserve power (see the
Standby Mode section).
Standby Mode
The MAX1887/MAX1897 slave controllers enter a low-
power standby mode when the ILIM voltage (V ILIM )
drops below 250mV (Table 4). Similar to shutdown
mode, standby forces DL high, pulls DH low, and dis-
ables the PWM controller to inhibit switching; however,
the bias and fault protection circuitry remain active so
the MAX1887/MAX1897 can continuously monitor the
ILIM input. When V ILIM is driven above 250mV, the
PWM controller is enabled.
When the slave controller ’ s current-limit voltage (V ILIM )
is set through a resistive divider between the master
controller ’ s reference and GND (see the Current-Limit
Circuitry section), the MAX1887/MAX1897 automatically
enters low-power standby mode when the master con-
troller shuts down. As the master ’ s reference powers
down, the resistive divider pulls ILIM below 250mV,
automatically activating the MAX1887/MAX1897 ’ s low-
power standby mode.
Current-Limit Circuitry
When the master ’ s inductor current exceeds its valley
current limit, the master extends its off time by forcing
DL high until the inductor current falls below the current-
limit threshold. Without a transition on the master ’ s low-
side gate driver, the slave cannot initiate a new on-time
pulse so the slave ’ s inductor current ramps down as
well, maintaining the current balance. Therefore, the
slave ’ s valley current limit only needs to protect the
slave controller if the current-balance circuitry or the
master current limit fails. The slave ’ s ILIM input voltage
should be selected to properly adjust the master ’ s cur-
rent-limit threshold.
Dual-Mode ILIM Input
The current-limit input (ILIM) features dual-mode opera-
tion, serving as both the standby mode control input
and the current-limit threshold adjustment. The slave
controller enters a low-power standby mode when the
ILIM voltage (V ILIM ) is pulled below 250mV. For ILIM
voltages from 400mV to 1.5V, the current-limit threshold
voltage is precisely 0.1 ? V ILIM . The current-limit volt-
age may be accurately set with a resistive voltage-
divider between the master controller ’ s reference and
GND, allowing the MAX1887/MAX1897 to automatically
enter the low-power standby mode.
Slave Current Limit
The slave current-limit circuit employs a unique “ valley ”
current-sensing algorithm. If the current-sense signal is
above the current-limit threshold, the MAX1887/ MAX1897
will not initiate a new cycle (Figure 3). The actual peak
inductor current is greater than the current-limit threshold
by an amount equal to the inductor ripple current.
Therefore, the exact current-limit characteristic and maxi-
mum load capability are a function of the current-limit
threshold, inductor value, and input voltage. The reward
for this uncertainty is robust, overcurrent sensing. When
combined with master controllers that contain output
undervoltage protection circuits, this current-limit method
is effective in almost every circumstance.
16
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