MAX1956ETI Maxim Integrated Products, MAX1956ETI Datasheet - Page 13

• Manufacturer Part Number: MAX1956ETI
• Description: DC/DC Switching Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1956ETI

• Number Of Outputs: 2
• Output Voltage: 0.8 V to 4.95 V
• Input Voltage: 1.6 V to 5.5 V
• Package / Case: TQFN EP-28
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

An external clock of 1080kHz to 1320kHz at SYNC
forces the controller to switch at half of this clock fre-
quency. DH1 and DH2 positive-going edges alternately
synchronize to the rising edge of the external clock,
thus operating 180° out-of-phase with each other. See
the Synchronization and Switching Waveforms in the
Typical Operating Characteristics.
The MAX1955/MAX1956 feature a low-power shutdown
mode that reduces the IC’s current consumption to less
than 20µA. For normal operation, connect EN to IN. To
place the part in low-current shutdown mode, connect
EN to GND.
When the MAX1955/MAX1956 enter shutdown (EN
goes low), soft-stop begins. In soft-stop, the output volt-
age is ramped down by lowering the FB_ regulation
voltage to zero in 80 steps of 10mV. Total soft-stop time
is typically 4.27ms.
Each controller can be shut down individually by
pulling COMP_ to GND with an open collector NPN
transistor (Figure 6). This shuts down the controller
immediately without going through soft-stop. Once
COMP_ is released, the controller powers up without
going through soft-start. To protect against inrush cur-
rent when using this power-up/-down method, use fold-
back current limit. Also, connect SEQ to GND to
prevent output 2 from powering down when the voltage
on output 1 drops.
In an effort to improve quality, many OEMs are testing
their system’s operation over the range of minimum and
maximum supply voltage. To facilitate this testing, the
MAX1955/MAX1956 have a voltage-margining feature
that increases or decreases the output voltages by 4%.
The voltage on EN controls voltage margining. To
increase the output voltage by 4%, apply (2/3) V
To reduce the output voltage by 4%, apply (1/3) V
One easy way to use the voltage-margining feature is to
make two control logic inputs (CTL1 and CTL2) by con-
necting two resistors to EN. Connect a 200kΩ resistor
from EN to CTL1, and a 100kΩ resistor from EN to CTL2
(Figure 5). The voltage margining is then controlled by
connecting CTL1 and CTL2 to IN or GND, as shown in
Table 1. Before applying voltage-margining, pull VCTL1
and VCTL2 to > V
Shutdown and Output Voltage
IN
______________________________________________________________________________________
180° Out-of-Phase Step-Down Controllers
- 0.5V to ensure proper startup.
1.6V to 5.5V Input, 0.5% Accurate, Dual
Synchronization
Margining (EN)
IN
IN
to EN.
to EN.
Thermal-overload protection limits total power dissipa-
tion of the MAX1955/MAX1956. When the junction tem-
perature exceeds +160°C, an internal thermal sensor
shuts down the device, allowing the IC to cool. The
thermal sensor turns the device on after the junction
temperature cools by 15°C. In a continuous thermal-
overload condition, this results in a pulsed output.
Under most operating conditions, the low-side MOSFET
conducts only positive inductor currents that flow from
source to drain and 1/2 of the inductor peak-to-peak
ripple current (~15% full load current) in the negative
direction when output is at no load. If the MAX1955/
MAX1956 are disabled before their soft-start cycle is
complete (~4ms), the converter is disabled without a
soft-stop, and the output discharges through its load. In
this case, if the converter is reenabled before the out-
put capacitor discharges completely, the soft-start
cycle resets the reference input to the error amp to zero
and ramps up again.
The converter forces DL on until the feedback drops
below the reference input. If the output is almost fully
charged when the converter turns back on, a large
negative current can build up in the inductor. If the
negative current is excessive, a high LX voltage spike
can occur because of parasitic circuit inductances as
DL is released. This high LX voltage spike can shut
down and latch off the circuit. To prevent this from hap-
pening, add a series resistor between DL and the gate
of the low-side MOSFET (Figure 6) to slow down the
turn-off di/dt, reducing the voltage spike and prevent-
ing the circuit from shutting down. A 1Ω resistor works
fine for most applications without noticeable degrading
impact on efficiency or Cdv/dt-induced turn-on effect.
Table 1. Voltage Margining
CTL1
V
V
0
0
IN
IN
Thermal-Overload Protection
Low-Side MOSFET Negative-
CTL2
V
V
0
0
IN
IN
Current Conduction
(2/3)V
(1/3)V
V
EN
0
IN
IN
IN
Shutdown
OUTPUT
Nominal
+4%
-4%
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