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参数资料
| 型号： | LM2631MTCX-ADJ/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 400 kHz SWITCHING FREQ-MAX, PDSO20 |
| 封装： | TSSOP-20 |
| 文件页数： | 11/12页 |
| 文件大小： | 265K |
| 代理商： | LM2631MTCX-ADJ/NOPB |
Operation (Continued)
when the duty cycle is higher than 50%. For the LM2631, this
compensation ramp is internally set to equal the maximum
down slope of the current amplifier output:
Where n = 5 is the gain of the current sense amplifier. The
maximum output voltage equals 6V. Also, a 10 H inductor
and a 0.025
sense resistor are assumed to determine the
internal compensation ramp. Different values of inductor and
sense resistor can be used as long as the resulted M
DOWN
(= nxR
SEN xVOUT/L) is less than MC.
Pulse-Skipping Mode at Light Load
Pulse-skipping mode can be enabled by pulling PFWM pin
low. This mode decreases switching frequency at light loads
to reduce the switching frequency related losses. If PFWM is
set at low, the controller goes into the pulse-skipping mode
when the sensed inductor current goes below the 25 mV
threshold set by the pulse-skipping comparator. In the
pulse-skipping mode, the high-side switch only turns on at
the beginning of a clock cycle when the voltage at the feed-
back pin falls below the reference voltage. Once the switch is
on, it stays on until the sensed current rises to the 25 mV
threshold
Fast Transient Response
When the output voltage fails to exceed 97% of the nominal
level, the low voltage regulation(LREG) comparator will set
the PWM logic to turn the high-side switch on at maximum
duty cycle. This improves transient response since it by-
passes the error amplifier and PWM comparator. During
start-up, the LREG is disabled.
Boost High-Side Gate Drive
A flying capacitor is used to bootstrap the power supply for
the high-side driver as illustrated in
Figure 1. The boost ca-
pacitor is charged from an internal voltage rail (about 5.5V)
through an internal diode when the synchronous rectifier
(low-side MOSFET) is on, and then boosts up the high-side
gate voltage to turn high-side MOSFET on at the beginning
of next cycle. The internal diode connecting between the VIN
pin and the CBOOT pin reduces the count of external com-
ponents. For low input voltage application (Vin < 5V), some
external charge pump circuitry can be used to boost the gate
voltage in order to reduce conduction loss. Details will be
discussed in the Application Circuits Section.
Supply Voltage for the LM2631
When 5V is available, it is recommended to connect LM2631
V
IN (pin13) to 5V. This can improve efficiency (see the sec-
ond figure in Typical Performance Characteristics), and also
reduce power dissipation inside the IC. Since the 5V supply
is only used to power the LM2631 (including the gate charge
for the external MOSFETs), it only requires a small amount
of current.
Reference
The 1.238V reference is of ±2.4% accuracy over tempera-
ture. A 220 pF capacitor is recommended between the V
REF
pin and ground. The load at the V
REF pin should not exceed
100A.
Frequency Control Pin (FADJ) and SYNC Pin
With the FADJ pin open, the switching frequency is 200 kHz.
The frequency can be increased by connecting a resistor be-
tween FADJ and ground. The device can also be synchro-
nized with an external CMOS or TTL logic clock in the range
from 200 kHz to 400 kHz. It is recommended to connect the
SYNC pin to ground if not used.
Protections
The current limit comparator provides the cycle-by-cycle cur-
rent limit function by turning off the high-side MOSFET
whenever the sensed current reaches 110 mV. Note that, the
current limit voltage will increase when output voltage is less
than 1.5V. A second level of current limit is accomplished by
the 80% low voltage detector: if the load pulls the output volt-
age down below 80% of the nominal value, the device will
turn off the high-side MOSFET and turn on the low-side
MOSFET. The overvoltage protection comparator will turn on
the low-side switch when the output voltage exceeds 120%
of the nominal value. Both protection features are disabled
during start-up. All latched conditions can be reset by shut-
ting the device down and then powering it up. Built-in under-
voltage lockout circuit will keep most of the internal function
blocks off until the input voltage rises to about 3.5V.
Soft Start
A capacitor at the SS pin provides the soft start feature.
When the regulator is first powered up, or when the SD pin
goes high, a 10A current source charges up the SS capaci-
tor from the 0.6V clamping voltage. The switch duty cycle
starts with narrow pulses and gradually get wider as the SS
pin voltage ramps up to about 1.3V, above which the duty
cycle will be controlled by the maximum current limit until the
output voltage rises to the nominal value and the regulator
starts to operate in the normal current mode PWM control.
The LM2631 use a digital counter, referenced to the oscilla-
tor frequency, to set the soft start timeout. The timeout is de-
pendent on the switching frequency (timeout = 4096/F
S). If
the output voltage doesn’t move within the ±3% window of
the nominal value during this period, the device will latch it-
self off.
Power Good
The LM2631 provides a power good signal by monitoring the
voltage at the FB pin and compared the feedback voltage
with the V
REF voltage. Once the output voltage exceeds the
±9% window of the nominal value, the PGOOD pin goes low,
and stays low until the output voltage returns to the ±3%
window of the nominal value.
Design Procedure
Guidelines for selecting external components are discussed
in this section.
Inductor Selection
The most critical parameters for the inductor are the induc-
tance, peak current and the dc resistance. The inductance is
related to the switching frequency and the ripple current:
Higher switching frequency allows smaller inductor, but re-
duces the efficiency. A higher value of ripple current reduces
inductance, but increase the conductance loss, core loss,
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