Datasheet

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On the rising edge of EN both controllers enter soft-
start. Soft-start gradually ramps up to the reference
voltage seen by the error amplifier in order to control
the outputs’ rate of rise and reduce input surge cur-
rents during startup. The soft-start period is 1024 clock
cycles (1024/f
SW
), and the internal soft-start DAC
ramps up the voltage in 64 steps. The output reaches
regulation when soft-start is completed. On the falling
edge of EN both controllers simultaneously enter soft-
stop, which reverses the soft-start ramp. The part
enters shutdown after soft-stop is complete.
Output-Voltage Sequencing
After the startup circuitry enables the controller, the
MAX1858 begins the startup sequence. Regulator 1
(OUT1) powers up with soft-start enabled. Once the first
converter’s soft-start sequence ends, Regulator 2 (OUT2)
powers up with soft-start enabled. Finally, when both con-
verters complete soft-start and both output voltages
exceed 90% of their nominal values, the reset output
(RST) goes high (see the Reset Output section). Soft-stop
is initiated by pulling EN low. Soft-stop occurs in reverse
order of soft-start, allowing last-on/first-off operation.
Reset Output
RST is an open-drain output. RST pulls low when either
output falls below 90% of its nominal regulation voltage.
Once both outputs exceed 90% of their nominal regula-
tion voltages and both soft-start cycles are completed,
RST goes high impedance. To obtain a logic-voltage out-
put, connect a pullup resistor from RST to the logic sup-
ply voltage. A 100kΩ resistor works well for most appli-
cations. If unused, leave RST grounded or unconnected.
Clock Synchronization (SYNC, CKO)
SYNC serves two functions: SYNC selects the clock out-
put (CKO) type used to synchronize slave controllers, or
it serves as a clock input so the MAX1858 can be syn-
chronized with an external clock signal. This allows the
MAX1858 to function as either a master or slave. CKO
provides a clock signal synchronized to the MAX1858’s
switching frequency, allowing either in-phase (SYNC =
GND) or 90° out-of-phase (SYNC = V
L
) synchronization
of additional DC-DC controllers (Figure 5). The
MAX1858 supports the following three operating modes:
• SYNC = GND: The CKO output frequency equals
REG1’s switching frequency (f
CKO
= f
DH1
) and the
CKO signal is in phase with REG1’s switching fre-
quency. This provides 2-phase operation when syn-
chronized with a second slave controller.
• SYNC = V
L
: The CKO output frequency equals two
times REG1’s switching frequency (f
CKO
= 2f
DH1
)
and the CKO signal is phase shifted by 90° with
respect to REG1’s switching frequency. This pro-
vides 4-phase operation when synchronized with a
second MAX1858 (slave controller).
• SYNC Driven by External Oscillator: The controller
generates the clock signal by dividing down the
SYNC input signal, so the switching frequency equals
half the synchronization frequency (f
SW
= f
SYNC
/2).
REG1’s conversion cycles initiate on the rising edge
of the internal clock signal. The CKO output frequen-
cy and phase match REG1’s switching frequency
(f
CKO
= f
DH1
) and the CKO signal is in phase. Note
that the MAX1858 still requires R
OSC
when SYNC is
externally clocked and the internal oscillator frequen-
cy should be set to 50% of the synchronization fre-
quency (f
OSC
= 0.5f
SYNC
).
Thermal Overload Protection
Thermal overload protection limits total power dissipation
in the MAX1858. When the device’s die-junction tempera-
ture exceeds T
J
= +160°C, an on-chip thermal sensor
shuts down the device, forcing DL_ and DH_ low, allow-
ing the IC to cool. The thermal sensor turns the part on
again after the junction temperature cools by 10°C.
During thermal shutdown, the regulators shut down, RST
goes low, and soft-start is reset. If the V
L
linear-regulator
output is short-circuited, thermal-overload protection is
triggered.
MAX1858
Dual 180° Out-of-Phase PWM Step-Down
Controller with Power Sequencing and POR
V
L
BST_
DH_
LX_
5
Ω
INPUT
(V
IN
)
MAX1858
Figure 3. Reducing the Switching-Node Rise Time