lm27213mtd National Semiconductor Corporation, lm27213mtd Datasheet - Page 14

• Manufacturer Part Number: lm27213mtd
• Description: Single Phase Hysteretic Buck Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Applications Information
Figure 3 below shows the theoretical waveform that is to be
expected at the CMP pin. Zero output ripple voltage is as-
sumed. In reality these signals ride on top of the regulator’s
output ripple and may be very hard to discern. There’s a
small delay time from the instant the voltage on the CMP pin
crosses the voltage on CMPREF. This delay will result in the
inductor peak current overshooting the hysteresis setting.
For high step-down ratios the inductor current down-slope
will be much more shallow than the up-slope. Therefore, the
undershoot magnitude will be less than the overshoot mag-
nitude. As a result of these delays the actual hysteresis will
be somewhat greater than programmed.
The actual wave shapes will be very dependant on the type
of output capacitor selected. The resistive component of
electrolytic type capacitors (ESR) will serve to provide a
significant amount of instantaneous feedforward due to the
current flow through the capacitors. By contrast, if an all
ceramic output capacitor decoupling network is employed,
the current flow through the capacitor is integrated over time,
and current information is phase shifted. This tends to alter
the regulator’s behavior somewhat. In particular, the operat-
ing frequency will be very hard to predict since decoupling
parasitics play a significant part in shaping the waveforms at
the CMP pin. As such, it is generally simplest to choose the
final value for the hysteresis resistor empirically.
The approximate operating frequency is:
FIGURE 2.
FIGURE 3.
(Continued)
20154323
20154322
14
Where:
R
L is the inductor value in µH
∆I
R
This equation is greatly simplified and fails to account for the
effects of output ripple, comparator delay times etc. There
are far too many subtle variables involved in this topology to
be able to make very accurate operating frequency esti-
mates. This equation should provide a ballpark starting point
with the final value of the hysteresis resistor arrived at em-
pirically. At high frequency the delays through the compara-
tor and driver will be the dominant factor in determining the
operating frequency.
CURRENT LIMITING
Current limit is achieved by comparing the instantaneous
voltage across the current sense resistor with the voltage
developed across the current limit threshold resistor, R13 in
the typical application circuit. If the controller sees this
threshold exceeded, the current switching cycle is termi-
nated and the hysteresis current dropped in half. The voltage
across the current limit set point resistor is determined by the
value of the resistor and the magnitude of the current
through the ILIMREF pin. This current is nominally three
times the current drawn through the V1R7 reference pin.
When current limit is reached, this current is reduced to
250% of the V1R7 current. Be sure to use the full load DC
current plus
mining the required current sense threshold.
POWER GOOD
The output voltage is sensed at the Sense pin (16) and
monitored by a window comparator with thresholds set to
nominally 88% and 112% of the selected output voltage set
point. As long as V
good signal will be logic high. This output is an open drain
device and requires an external pull up. At power up, the
LM27213 will wait approximately 5ms after XPOK is as-
serted before releasing PGOOD. If the output voltage is then
within the
OVER-VOLTAGE PROTECTION
The sense pin is also used to provide the input to the
over-voltage protection circuitry. If at any time the output is
determined to be more than a nominal 120% of the voltage
set on the VOVP pin (17), the high-side FET is turned off and
the low-side FET is turned on. The soft start capacitor will
also be discharged. This state is latched. In order to initiate
a restart, remove and restore power to the controller or
toggle the VRON pin.
SOFT START
When VRON is enabled the regulator begins a normal start
sequence that actively controls the rise of output voltage. An
internal 20µA current source supplies current through the
Soft Start pin (25) that charges the soft start capacitor. The
output voltage is forced to track this voltage up. The result is
a linear output voltage rise. The capacitor value required is
s
hys
hyst
is the value of the current sense resistor in Ohms
is the value of the hysteresis resistor in Ohms
is the hysteresis current in Amps
±
12% window, the flag will be asserted.
1
⁄
2
the pk-pk inductor ripple current when deter-
core
remains within the window, the power