LM27212SQX/NOPB National Semiconductor, LM27212SQX/NOPB Datasheet - Page 16

• 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

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Design Considerations
Example: soft start time is preferred to be 1ms to 3ms, initial
Vcore is 1.37V, and Dynamic VID / Mode Change slew rate
is preferred to be no less than 5mV/µs.
So,
Double-checking the soft start time:
To make sure during Sleep entry PGOOD doesn’t go low, SS
voltage must hit the PGOOD window of the target voltage
before the PGOOD mask timer expires (around 130 µs). The
following equation can be used to establish the approximate
value of the largest SS capacitor.
where ∆
0.74V) and the SS-pin voltage immediately before the Sleep
entry, and t is the time it tkaes to reach the VSLP voltage. A
and B are coefficients that depend on temperature.
It is found that the SS-pin sink current is the lowest at 110˚C
over the temperature range of -5˚C to 110˚C.
SETTING VOVP, VBOOT, VSLP AND IH
Refer to the Typical Application circuit.
The hysteresis current ih used in the previous equations is
equal to the current sourced by the V1R7 pin. So calculation
of the R2, R3, R5 and R6 values is straightforward.
Example: hysteresis current ih = 100µA, Sleep voltage VSLP
= 0.748V, initial start up voltage VBOOT = 1.37V, OVP
threshold VOVP = 1.7V.
0
Temperature
is the difference between VSLP (typically around
Room
110˚C
-240e-6
-220e-6
A
(Continued)
-145e-6
-90e-6
B
16
SETTING STOP CPU VOLTAGE VSTP
Refer to Typical Application circuit.
The Stop CPU voltage VSTP is a certain percentage lower
than the DAC output voltage. The equation used to deter-
mine the R7 resistor values is:
where δ is the percentage VSTP is lower than VDAC.
Example: δ = -2.69%.
If we choose R8 = 100kΩ, then
Choosing too small an R8 will result in too much current
draw from the VDD pin, hurting system efficiency. Too large
an R8 value may result in noise issues.
SETTING THE HYSTERESIS
Refer to Figure 1. The hysteresis voltages across RH1 and
RH2 contribute to switching frequency characteristics, induc-
tor ripple current, noise immunity and line regulation. Typi-
cally the higher the hysteresis, the lower the switching fre-
quency and the tighter the frequency range. Also the higher
the hysteresis, the higher the ripple current. In a typical
mobile CPU design, the hysteresis is typically set at a few
milivolts.
Example: hysteresis voltage is set at 6mV, hysteresis current
ih = 100µA
So RH1 = RH2 = 6mV ÷ 100µA = 60Ω.