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PDF LM3477 Data sheet ( Hoja de datos )
| Número de pieza | LM3477 | |
| Descripción | High Efficiency High-Side N-Channel Controller for Switching Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LM3477 (archivo pdf) en la parte inferior de esta página. Total 23 Páginas | ||
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June 2002
LM3477/LM3477A
High Efficiency High-Side N-Channel Controller for
Switching Regulator
General Description
The LM3477/A is a high-side N-channel MOSFET switching
regulator controller. It can be used in topologies requiring a
high side MOSFET such as buck, inverting (buck-boost) and
zeta regulators. The LM3477/A’s internal push pull driver
allows compatibility with a wide range of MOSFETs. This, the
wide input voltage range, use of discrete power components
and adjustable current limit allows the LM3477/A to be opti-
mized for a wide variety of applications.
The LM3477/A uses a high switching frequency of 500kHz to
reduce the overall solution size. Current-mode control re-
quires only a single resistor and capacitor for frequency
compensation. The current mode architecture also yields
superior line and load regulation and cycle-by-cycle current
limiting. A 5µA shutdown state can be used for power sav-
ings and for power supply sequencing. Other features in-
clude internal soft-start and output over voltage protection.
The internal soft-start reduces inrush current. Over voltage
protection is a safety feature to ensure that the output volt-
age stays within regulation.
The LM3477A is similar to the LM3477. The primary differ-
ence between the two is the point at which the device
transitions into hysteretic mode. The hysteretic threshold of
the LM3477A is one-third of the LM3477.
Features
n 500kHz switching frequency
n Adjustable current limit
n 1.5% reference
n Thermal shutdown
n Frequency compensation optimized with a single
capacitor and resistor
n Internal softstart
n Current mode operation
n Undervoltage lockout with hysteresis
n 8-lead Mini-SO8 (MSOP-8) package
Applications
n Local Voltage Regulation
n Distributed Power
n Notebook and Palmtop Computers
n Internet Appliances
n Printers and Office Automation
n Battery operated Devices
n Cable Modems
n Battery Chargers
LM3477
LM3477A
Hysteretic
Threshold
) 36% of
programmed current
limit
) 12% of
programmed current
limit
*See Hysteretic Threshold and section for more information.
Typical Application Circuit
20003333
Typical High Efficiency Step-Down (Buck) Converter
© 2002 National Semiconductor Corporation DS200033
www.national.com
1 page  
Electrical Characteristics (Note 4) (Continued)
Specifications in Standard type face are for TJ = 25˚C, and in bold type face apply over the full Operating Temperature
Range. Unless otherwise specified, VIN = 12V.
Symbol
Parameter
Conditions
Typical
Limit
Units
VSD
Shutdown Threshold (Note 8)
Output = High
1.15
1.35
V
V (max)
Output = Low
0.65
V
0.3 V (min)
ISD
TSD
Shutdown Pin Current
Thermal Shutdown
VSD = 5V
VSD = 0V
−1
+1
165
µA
˚C
TSH Thermal Shutdown Hysteresis
θJA Thermal Resistance
MM Package
10
200
˚C
˚C/W
5 www.national.com
5 Page 
Functional Description (Continued)
The important differences between the LM3477 and the
LM3477A are summarized in Figure 1. The voltages in Fig-
ure 1 can be referred to the switch current by dividing
through by RSN. The LM3477A has a lower hysteretic thresh-
old voltage VHYS, and thus will operate in PWM mode for a
larger load range than the LM3477. Typically, VHYS = 32mV
for the LM3477, while VHYS = 11mV for the LM3477A. The
difference in area between the shaded regions give a graphi-
cal representation of this. The lightly shaded region is the
extra PWM operating area gained by using the LM3477A.
Thus the benefits of operating in PWM mode such as a well
regulated output voltage with low noise ripple are extended
to a larger load range when the LM3477A is used. While less
significant, the other noteworthy difference between the two
parts is in the short circuit current limit VSC.
VSC is a ceiling limit for the peak sense voltage VSNpk (see
the SHORT CIRCUIT CURRENT LIMIT section). VSC is
lower in the LM3477A than in the LM3477 (see the ELEC-
TRICAL CHARACTERISTICS section for limits).
Over Voltage Protection
The LM3477/A has over voltage protection (OVP) for the
output voltage. OVP is sensed at and is in respect to the
feedback pin (pin 3). If at anytime the voltage at the feedback
pin rises to VFB + VOVP, OVP is triggered. See ELECTRICAL
CHARACTERISTICS section for limits on VFB and VOVP.
OVP will cause the drive pin to go low, forcing the power
MOSFET off. With the MOSFET off, the output voltage will
drop. The LM3477/A will begin switching again when the
feedback voltage reaches VFB + (VOVP - VOVP(HYS)). See
ELECTRICAL CHARACTERISTICS for limits on VOVP(HYS).
OVP can be triggered by any event that causes the output
voltage to rise out of regulation. There are several common
circumstances in which this can happen, and it is beneficial
for a designer to be aware of these for debugging purposes,
since the mode of operation changes from the normal Pulse
Width Modulation (PWM) mode to the hysteretic mode. In
the hysteretic mode the output voltage is regulated between
a high and low value that results in a higher ripple magnitude
and lower ripple frequency than in the PWM mode, see
Figure 2.
200033C1
FIGURE 2. The Feedback Voltage is related to the
Output Voltage. See different Ripple Components in
PWM and Hysteretic Modes
If the load current becomes too low, the LM3477/A will
increase the duty cycle, causing the voltage to rise and
trigger the OVP. The reasons for this involve the way the
LM3477/A regulates the output voltage, using a control
waveform at the pulse width modulator. This control wave-
form has upper and lower bounds.
Another way OVP can be tripped is if the input voltage rises
higher than the LM3477/A is able to regulate in pulse width
modulation (PWM) mode. The output voltage is related to
the input voltage by the duty cycle as: VOUT = VIN*D. The
LM3477/A has a minimum duty cycle of 16.5% (typical), due
to the blank-out timing, TMIN. If the input voltage increases
such that the duty cycle wants to be less than DMIN, the duty
cycle will hold at DMIN and the output voltage will increase
with the input voltage until it trips OVP.
It is useful to plot the operational boundaries in order to
illustrate the point at which the device switches into hyster-
etic mode. In Figure 1, the limits shown are with respect to
the peak voltage across the sense resistor RSN, (VSNpk);
they can be referred to the peak inductor current by dividing
through by RSN. VSNpk is bound to the shaded regions. In
normal circumstances VSNpk is required to be in the shaded
region, and the LM3477/A will operate in the PWM mode. If
operating conditions are chosen such that VSNpk would not
normally fall in the shaded regions, then the mode of opera-
tion is changed so that VSNpk will be in the shaded region,
and the part will operate in the hysteretic mode. What actu-
ally happens is that the LM3477/A will not allow VSNpk to be
outside of the shaded regions, so the duty cycle is adjusted.
The output voltage transient response overshoot can also
trigger OVP. As discussed in the OUTPUT CAPACITOR
section, if the capacitance is too low or ESR too high, the
output voltage overshoot will rise high enough to trigger
OVP. However, as long as there is room for the duty cycle to
adjust (the converter is not near DMIN or DMAX), the
LM3477/A will return to PWM mode after a few cycles of
hysteretic mode operation.
There is one last way that OVP can be triggered. If the
unregulated input voltage crosses 7.2V, the output voltage
will react as shown in Figure 3. The internal bias of the
LM3477/A switches supplies at 7.2V. When this happens, a
sudden small change in bias voltage is seen by all the
internal blocks of the LM3477/A. The control voltage, VC,
shifts because of the bias change, the PWM comparator
tries to keep regulation. To the PWM comparator, the sce-
nario is identical to step change in the load current, so the
response at the output voltage is the same as would be
observed in a step load change. Hence, the output voltage
overshoot here can also trigger OVP. The LM3477/A will
regulate in hysteretic mode for several cycles, or may not
recover and simply stay in hysteretic mode until the load
current drops. Note that the output voltage is still regulated in
hysteric mode. Predicting whether or not the LM3477/A will
come out of hysteretic mode in this scenario is a difficult
task, however it is largely a function of the output current and
the output capacitance. Triggering hysteretic mode in this
way is only possible at higher load currents. The method to
avoid this is to increase the output capacitance.
11 www.national.com
11 Page | ||
| Páginas | Total 23 Páginas | |
| PDF Descargar | [ Datasheet LM3477.PDF ] | |
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