LM27212SQX/NOPB National Semiconductor, LM27212SQX/NOPB Datasheet - Page 13

• Manufacturer Part Number: LM27212SQX/NOPB
• Description: IC CURR-MODE BUCK CTRLR 48-LLP
• Manufacturer: National Semiconductor
• Type: Step-Down (Buck)r

Specifications of LM27212SQX/NOPB

• Internal Switch(s): No
• Synchronous Rectifier: Yes
• Number Of Outputs: 2
• Voltage - Output: 0.7 ~ 1.71 V
• Voltage - Input: 5 ~ 30 V
• Operating Temperature: -5°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 48-LLP
• Power - Output: 1.56W
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Frequency - Switching: -
• Other names: LM27212SQX; LM27212SQXTR

Operation Description
CURRENT SHARING
Current sharing is guaranteed by actively sensing the induc-
tor current in each channel and comparing the peak of each
sensed current with the same reference. In a current mode
hysteretic controller such as the LM27212, current sharing is
intrinsic. However, due to the low resistance value of the
sense resistors (as low as 1mΩ), care should be exercised
to make sure that the layout of the sense resistors is sym-
metrical, especially how the sense lines are connected to the
sense resistors.
CURRENT LIMITING
An adjustable current limit is built in. An internal current
flowing from the ILIMREF pin to the output through a resistor
establishes a voltage which is compared with the voltage
across the sense resistors to determine whether the sense
resistors are conducting too much current.
When the peak inductor current in Channel 1 exceeds the
preset limit, the OUT1 pin will go low, causing the inductor
current to drop. When the inductor current drops by an
amount that corresponds to the hysteresis of the current
limit, the OUT2 pin will be allowed to go high. If the inductor
current of Channel 2 also hits current limit, then OUT2 pin
will go low and so Channel 2 current will fall. When Channel
2 current falls by an amount that corresponds to current limit
hysteresis, the OUT1 pin is allowed to go high again.
In the case of a persistent over current, the output voltage
will continue to droop until the load current is equal to the
current limit value. If the output voltage droops too much
(12% below nominal), PGOOD will be de-asserted and the
system may use that to de-assert VRON and thus shut down
the regulator.
Note:
1. DEM stands for Diode Emulator Mode.
2. Only for a transition from 000 to 100, a 130µs 2-phase operation is
enforced.
SLP STP_CPU#
0
0
0
0
1
1
1
1
Modes During Normal Operation
0
0
1
1
0
0
1
1
DE_EN#
0
1
0
1
0
1
0
1
Vcore = VID - offset
Vcore = VID - offset
Mode Description
Ch2 = Asynch.
Vcore = VSLP
Vcore = VSLP
(Continued)
2-ph, Synch.
2-ph, Synch.
2-ph, Synch.
Vcore = VID
Ch1 = DEM
Ch1 = DEM
Ch2 = off
13
Design Considerations
NOMENCLATURE
ESR – Equivalent Series Resistance;
ESL - Equivalent Series Inductance;
Loading transient – a load transient when the load current
goes from minimum load to full load;
Unloading transient – a load transient when the load current
goes from full load to minimum load;
C
C
D – duty cycle;
f – switching frequency;
r – load line slope, e.g. -3mV/A or -3mΩ;
∆V
load transient, as derived from load device specifications;
∆I
load device manufacturer;
V
GENERAL
Due to the large and ultra-fast load transient behavior in
modern digital devices, it is typically easier to start the de-
sign process with the output capacitors.
SWITCHING FREQUENCY RANGE
In a current-mode hysteretic controller such as the
LM27212, switching frequency can be rather complicated to
calculate. If we assume that the ESR zero frequency is much
lower than the typical switching frequency (typically true for
non-MLCs), the switching frequency can be determined from
the following equation (refer to Figure 1):
Where
rip
min
max
c_s
c_s
– peak-to-peak output voltage ripple;
– minimum allowed output capacitance;
– maximum allowed output capacitance;
– maximum load current change, as specified by the
– maximum allowed output voltage excursion during a
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