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PDF MIC2174 Data sheet ( Hoja de datos )
| Número de pieza | MIC2174 | |
| Descripción | Synchronous Buck Controller | |
| Fabricantes | Micrel Semiconductor | |
| Logotipo |  |
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MIC2174/MIC2174C
Synchronous Buck Controller
Featuring Adaptive On-Time Control
40V Input, 300kHz
Hyper Speed Control™ Family
General Description
Features
The Micrel MIC2174/MIC2174C is a fixed-frequency,
synchronous buck controller featuring adaptive on-time
control. The MIC2174/MIC2174C operates over an input
supply range of 3V to 40V at a fixed switching frequency of
300kHz and is capable of driving 25A of output current.
The output voltage is adjustable down to 0.8V.
A unique Hyper Speed Control™ architecture allows for
ultra-fast transient response while reducing the output
capacitance. It also makes ultra-fast transient response
while reducing the output capacitance and also allows for
extremely low duty-cycle operation. The MIC2174 /
MIC2174C utilizes an adaptive TON ripple controlled
architecture. A UVLO is provided to ensure proper
operation under power-sag conditions to prevent the
external power MOSFET from overheating. A soft-start is
provided to reduce inrush current. Foldback current limit
and “hiccup” mode short-circuit protection ensure FET and
load protection.
The MIC2174/MIC2174C is available in a 10-pin MSOP
(MAX1954A-compatible) package with an operating
junction temperature range from –40°C to +125°C.
All support documentation can be found on Micrel’s web
site at: www.micrel.com.
• Hyper Speed Control™ architecture enables:
– High delta V operation (VHSD = 40V and VOUT = 0.8V)
– Smaller output capacitors than competitors
• 3V to 40V input voltage
• Any CapacitorTM stable
- Zero ESR to high ESR
• 300kHz switching frequency
• Adjustable output from 0.8V to 5.5V (VHSD ≤ 28V)
• Adjustable output from 0.8V to 3.6V (VHSD > 28V)
• ±1% FB accuracy (MIC2174)
• ±3% FB accuracy (MIC2174C)
• Up to 94% efficiency
• Foldback current limit and “hiccup” mode short-circuit
protection
• Thermal shutdown
• Safe start-up into pre-biased loads
• –40°C to +125°C junction temperature range
Applications
• Telecom Networking
• Industrial Equipment
• Distributed DC power systems
____________________________________________________________________________________________________________
Typical Application
Synchronous Buck Controller Featuring Adaptive On-Time Control
100
95
90
85
80
75
70
65
60
55
50
0
12V to 3.3V Efficiency
VIN=5V
2468
OUTPUT CURRENT (A)
10
Hyper Speed Control and Any Capacitor are trademarks of Micrel, Inc.
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
September 2010
M9999-091310-C
1 page  
Micrel, Inc.
Electrical Characteristics(4)
VBST − VLX = 5V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter
Soft-Start
Condition
Soft-Start Period
Oscillator
Switching Frequency
Maximum Duty Cycle
MIC2174
MIC2174C
Measured at DH (7)
MIC2174C
Minimum Duty Cycle
Measured at DH, VFB = 1V
FET Drives
DH, DL Output Low Voltage
ISINK = 10mA
DH, DL Output High Voltage
ISOURCE = 10mA
DH On-Resistance, High State
DH On-Resistance, Low State
DL On-Resistance, High State
DL On-Resistance, Low State
LX Leakage Current
HSD Leakage Current
Thermal Protection
Over-Temperature Shutdown
Over-Temperature Shutdown
Hysteresis
VLX = 40V, VIN = 5.5V,VBST = 45.5V
VLX = 40V, VIN = 5.5V,VBST = 45.5V
Shutdown Control
EN Logic Level Low
EN Logic Level High
EN Pull-Up Current
3V < VIN <5.5V
3V < VIN <5.5V
Note:
7. The maximum duty cycle is limited by the fixed mandatory off time TOFF of typical 363ns.
MIC2174/MIC2174C
Min.
Typ. Max. Units
6 ms
0.225
0.202
0.3 0.375 MHz
0.27 0.338
87
%
87
0%
0.1 V
VIN-0.1V
or
VBST-0.1V
V
2.1 3.3
Ω
1.8 3.3
Ω
1.8 3.3
Ω
1.2 2.3
Ω
55 µA
21 µA
155 °C
10 °C
0.4 0.8
V
0.9 1.2
V
50 µA
September 2010
5 M9999-091310-C
5 Page 
Micrel, Inc.
MIC2174/MIC2174C
Functional Description
The MIC2174/MIC2174C is an adaptive on-time
synchronous buck controller built for low cost and high
performance. It is designed for a wide input voltage
range from 3V to 40V and for high output power buck
converters. An estimated-ON-time method is applied in
MIC2174/MIC2174C to obtain a constant switching
frequency and to simplify the control compensation. The
over-current protection is implemented without the use of
an external sense resistor. It includes an internal soft-
start function which reduces the power supply input
surge current at start-up by controlling the output voltage
rise time.
Theory of Operation
The MIC2174/MIC2174C is an adaptive on-time
synchronous buck controller. Further, Figure 1 illustrates
the block diagram for the control loop. The output
voltage variation will be sensed by the
MIC2174/MIC2174C feedback pin FB via the voltage
divider R1 and R2, and compared to a 0.8V reference
voltage VREF at the error comparator through a low gain
transconductance (gm) amplifier, which improves the
MIC2174/MIC2174C converter output voltage regulation.
If the FB voltage decreases and the output of the gm
amplifier is below 0.8V, then the error comparator will
trigger the control logic and generate an ON-time period,
where in DH pin is logic high and DL pin is logic low. The
ON-time period length is predetermined by the “FIXED
TON ESTIMATION” circuitry:
TON(estimated)
=
VOUT
VHSD × 300kHz
(1)
where VOUT is the output voltage, VHSD is the power
stage input voltage.
After an ON-time period, the MIC2174/MIC2174C goes
into the OFF-time period. This is when the DH pin is
logic low and DL pin is logic high. The OFF-time period
length depends upon the FB voltage in most cases.
When the FB voltage decreases and the output of the
gm amplifier is below 0.8V, then the ON-time period is
triggered and the OFF-time period ends. If the OFF-time
period determined by the FB voltage is less than the
minimum OFF time TOFF(min), which is about 363ns
typical, then the MIC2174/MIC2174C control logic will
apply the TOFF(min) instead. TOFF(min) is required to
maintain enough energy in the Boost capacitor (CBST) to
drive the high-side MOSFET.
The maximum duty cycle is obtained from the 363ns
TOFF(min):
Dmax =
TS
− TOFF(min)
TS
=
1−
363ns
TS
where Ts = 1/300kHz = 3.33μs. It is not recommended to
use MIC2174/MIC2174C with a OFF-time close to
TOFF(min) during steady-state operation. Also, as VOUT
increases, the internal ripple injection will increase and
reduce the line regulation performance. Therefore, the
maximum output voltage of the MIC2174 should be
limited to 5.5V for up to 28V VHSD and 3.6V for VHSD
higher than 28V. If a higher output voltage is required,
use the MIC2176 instead. Please refer to “Setting Output
Voltage” subsection in “Application Information” for more
details.
The power stage input voltage VHSD is fed into the Fixed
TON Estimation block through a 6:1 divider and 5V
voltage clamper. Therefore, if the VHSD is higher than
30V, then the Fixed TON Estimation block uses 30V to
estimate TON instead of the real VHSD. As a result, the
switching frequency will be less than 300kHz:
fSW(VHDS >30V)
=
30V
VHSD
× 300kHz
(2)
The estimated ON-time method results in a constant
300kHz switching frequency up to 30V VHSD. The actual
ON-time varies with the different rising and falling times
of the external MOSFETs. Therefore, the type of the
external MOSFETs, the output load current, and the
control circuitry power supply VIN will modify the actual
ON-time and the switching frequency. Also, the minimum
TON results in a lower switching frequency in high VHSD
and low VOUT applications, such as 36V to 1.0V. The
minimum TON measured on the MIC2174/MIC2174C
evaluation board with Si7148DP MOSFETs is about
184ns. During the load transient, the switching frequency
is changed due to the varying OFF time.
To illustrate the control loop, the steady-state scenario
and the load transient scenario are analyzed. For easy
analysis, the gain of the gm amplifier is assumed to be 1.
With this assumption, the inverting input of the error
comparator is the same as the FB voltage. Figure 2
shows the MIC2174/MIC2174C control loop timing
during steady-state operation. During steady-state, the
gm amplifier senses the FB voltage ripple, which is
proportional to the output voltage ripple and the inductor
current ripple, to trigger the ON-time period. The ON-
time is predetermined by the estimation. The ending of
OFF-time is controlled by the FB voltage. At the valley of
September 2010
11 M9999-091310-C
11 Page | ||
| Páginas | Total 27 Páginas | |
| PDF Descargar | [ Datasheet MIC2174.PDF ] | |
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