MAX1887 Maxim Integrated Products, MAX1887 Datasheet - Page 16

• Manufacturer Part Number: MAX1887
• Description: (MAX1887 / MAX1897) Quick-PWM Slave Controllers
• Manufacturer: Maxim Integrated Products

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
and use standby mode to conserve power (see the
Standby Mode section).
The MAX1887/MAX1897 slave controllers enter a low-
power standby mode when the ILIM voltage (V
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
PWM controller is enabled.
When the slave controller’s current-limit voltage (V
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.
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-
Quick-PWM Slave Controllers for
Multiphase, Step-Down Supplies
Table 4. Operating Mode Truth Table
X = Don’t Care
16
SHDN
GND
V
V
______________________________________________________________________________________
CC
CC
(< 0.25V)
(> 0.25V)
GND
High
ILIM
X
ILIM
is driven above 250mV, the
Current-Limit Circuitry
Switching
High
High
DL
Standby Mode
Shutdown
Operation
Standby
Normal
MODE
ILIM
ILIM
CC
Micropower, shutdown mode (I
DH forced low, and the PWM controller disabled.
Low-power, standby mode (I
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.
Low-noise, fixed-frequency, PWM operation. The inductor current
reverses with light loads.
)
)
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.
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
voltages from 400mV to 1.5V, the current-limit threshold
voltage is precisely 0.1
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.
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.
ILIM
COMMENTS
CC
) is pulled below 250mV. For ILIM
CC
+ I
+I
DD
✕
DD
= 20µA typ). DL forced high,
V
< 1µA typ). DL forced high,
ILIM
. The current-limit volt-
Dual-Mode ILIM Input
Slave Current Limit