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PDF LM21305 Data sheet ( Hoja de datos )
| Número de pieza | LM21305 | |
| Descripción | 5A Adjustable Frequency Synchronous Buck Regulator | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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LM21305
PRELIMINARY
June 8, 2010
5A Adjustable Frequency Synchronous Buck Regulator
General Description
The LM21305 is a full featured adjustable frequency syn-
chronous buck regulator capable of delivering up to 5A of
continuous output current. The device is optimized to work
over the input voltage range of 3V to 18V making it suitable
for wide variety of applications. The LM21305 provides 1%
output-voltage accuracy and excellent line and fast load tran-
sient response for digital loads. The device offers flexible
system configuration via programmable switching frequency
and ability to synchronize switching frequency. The device
also provides internal soft-start to limit in-rush current, cycle-
by-cycle current limiting, and thermal shutdown.
The device features internal over voltage protection (OVP)
and over current protection (OCP) circuits for increased sys-
tem reliability. A precision enable pin and integrated UVLO
allows the turn-on of the device to be tightly controlled and
sequenced. Start-up inrush currents are limited by an internal
Soft-Start circuit. Fault detection and supply sequencing are
possible with the integrated power good circuit.
The frequency of this device can be adjusted from 300 kHz to
1.5 MHz through an external resistor. The LM21305 is offered
in a 28-pin LLP package.
Key Specifications
Features
■ Single rail input voltage from 3V to 18V
■ Feedback Voltage of 0.6V
■ 1% typical output voltage accuracy
■ High-efficiency switcher core
■ Switching frequency range: 300 kHz to 1.5 MHz
■ Resistor programmable switching frequency
■ Ability to synchronize switching frequency
■ Precision enable
■ Internal soft-start to reduce in-rush current
■ PGOOD function
■ Under Voltage Lock Out (UVLO)
■ Over Voltage Protection (OVP)
■ Fast transient response
■ Cycle-by-cycle current limiting
■ Thermal shutdown
■ LLP-28 package (5mm x 5mm x 0.8mm, 0.5mm pitch)
Applications
■ Point of Load regulation from 3.3V, 5V, and 12V rails
■ Power supply for DSPs, FPGAs, ASICs and Processors
■ Broadband, Networking and Optical Communications
Infrastructure
Typical Application Circuit
© 2010 National Semiconductor Corporation 301111
30111101
www.national.com
1 page  
Symbol
Parameter
Remarks
V2V5
Internal LDO2 output voltage
measured at 2V5 pin
COUT-cap-2V5
Recommended COUT
capacitance connected to 2V5
pin
Ceramic capacitor
Ishort-2V5
VFCBOOT-D
Short circuit current
CBOOT diode forward voltage
Measured between 5V0 and CBOOT
@ 10 mA
ICBOOT
Tstartup-delay
CBOOT Leakage Current
VCBOOT = 5.5V, VEN = 5V
Startup time from EN high to the
starting of the internal soft-start
(Note 9)
SS Internal soft-start (Note 9)
OSCILLATOR
FOSC-nom
Oscillator Frequency, nominal
measured at SW pin
FOSC-MAX
Maximum Oscillator Frequency
measured at SW pin
R_freq = 30 kΩ
FOSC-MIN
Minimum Oscillator Frequency
measured at SW pin
R_freq = 169 kΩ
TOFF-MIN
Minimum Off Time measured at Fs = 1.5 MHz, VIN = 3.3V, VFB = 1V,
SW pin
divider = 5.5
TON-MIN
Minimum On Time measured at
SW pin
Fs = 1.5 MHz, divider = 1
LOGIC
VIH-EN
EN Pin Rising Threshold
VHYST-EN
EN Pin Hysteresis Window
IEN-IN
EN Pin Input Current
VIH-UV-PGOOD PGOOD UV Rising Threshold
VHYST-UV-
PGOOD
PGOOD UV Hysteresis
Threshold
IOL- PGOOD
PGOOD sink current
IOH- PGOOD
PGOOD leakage current
THERMAL SHUTDOWN
VEN = 3.3V
with respect to VFB nom
with respect to VFB nom
VOL = 0.2V
VOH = 18V
TSD
TSD-HYS
Thermal Shutdown
Thermal Shutdown Hysteresis
(Note 9)
Min
300
Typ Max
2.47
100
45
0.83
3.55
160
1
750
1500
300
50
65
1500
1.2
100
94
−4
2
160
10
Unit
V
nF
mA
V
µA
µs
ms
kHz
kHz
kHz
ns
ns
V
mV
%
%
mA
nA
°C
°C
5 www.national.com
5 Page 
Operation Description
The LM21305 employs peak current mode control. The 0.6V
reference is compared to the feedback signal at the error amp.
The PWM modulator block compares the on-time current
sense information with the summation of the error amp output
(control voltage) and slope compensation. The PWM modu-
lator outputs on/off signals to the high side and low side
drivers. Adaptive dead time control is applied to the PWM
output such that no shoot through current exists. The drivers
then amplify the PWM signals to control the integrated high
side and low side MOSFETs.
SWITCHING REGULATOR
The LM21305 employs buck type (step down) architecture. It
utilizes many advanced features to achieve excellent voltage
regulation and efficiency. This easy to use regulator features
two integrated switches and is capable of supplying up to 5A
of continuous output current. The regulator utilizes peak cur-
rent mode control with slope compensation scaled with
switching frequency to optimize stability and transient re-
sponse over the entire output voltage and switching frequen-
cy range. Peak current mode control also provides inherent
line feed-forward, cycle-by-cycle current limiting and easy
loop compensation. The switching frequency can be adjusted
between 300 kHz and 1.5 MHz. The device can operate with
a small external L-C filter and still provide very low ripple volt-
age. The precision internal voltage reference allows the out-
put to be set as low as 0.6V. Using an external compensation
circuit, the regulator bandwidth can be selected based on the
switching frequency to provide fast load transient responses.
The switching regulator is specially designed for high effi-
ciency operation throughout the load range. Synchronous
rectification yields high efficiency for low voltage and high load
current situations, while optional DCM operation extends high
efficiency conversion to lower load currents. Fault protection
features include: high side and low side switch current limit-
ing, negative current limiting on the low side switch, over
voltage protection and thermal shutdown. The device is avail-
able in the LLP-28 package featuring an exposed pad to aid
thermal dissipation. The LM21305 can be used in numerous
applications to efficiently step-down from a wide range of rails:
3V to 18V.
PEAK CURRENT MODE CONTROL
In most applications, the peak current mode control architec-
ture used in the LM21305 only requires two external compo-
nents to achieve a stable design. The external compensation
allows the user to set the crossover frequency and phase
margin, thus optimize the transient performance of the device.
For duty cycles above 50% all current mode control buck
converters require the addition of an artificial ramp to avoid
sub-harmonic oscillation. This artificial linear ramp is com-
monly referred to as slope compensation. The amount of
slope compensation in LM21305 will automatically change
depending on the switching frequency: higher the switching
frequency, larger the slope compensation. This allows small-
er inductors to be used with high switching frequency and
increase the power density.
SWITCHING FREQUENCY SETTING AND
SYNCHRONIZATION
The switching regulator in LM21305 can operate at a fre-
quency ranging from 300 kHz to 1.5 MHz. The switching
frequency can be set / controlled by two ways. One is by se-
lecting the external resistor attached to the FREQ pin to set
the internal oscillator frequency, which controls the switching
frequency. An external 100 pF capacitor CFRQ should also
be connected from the FREQ pin to signal ground as a noise
filter, as shown in Figure 1.
30111104
FIGURE 1. Switching Frequency Set by External Resistor
The other way is to synchronize the switching action to an
external clock or other fixed frequency signals in the range of
300 kHz to 1.5 MHz. The external clock should be applied
through a 100 pF coupling capacitor, CFRQ, as shown in
Figure 2.
30111105
FIGURE 2. Switching Frequency Synchronized to External Clock
11 www.national.com
11 Page | ||
| Páginas | Total 28 Páginas | |
| PDF Descargar | [ Datasheet LM21305.PDF ] | |
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