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MIC2194
Datasheet (Micrel)
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Datasheet MIC2194 Datasheet (Micrel)

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Page 1
MIC2194 Micrel
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MIC2194
400kHz SO-8 Buck Control IC
Final Information
General Description
Micrel’s MIC2194 is a high efficiency PWM buck control IC housed in the SO-8 package. Its 2.9V to 14V input voltage range allows it to efficiently step down voltages in 3.3V, 5V, and 12V systems as well as 1- or 2-cell Li Ion battery powered applications. The flexible architecture of the MIC2194 allows for it to be configured as a buck or a buck-boost converter.
The MIC2194 solution saves valuable board space. The device is housed in the space-saving SO-8 package, whose low pin-count minimizes external components. Its 400kHz PWM operation allows a small inductor and small output capacitors to be used. The MIC2194 can implement all­ceramic capacitor solutions.
The MIC2194 drives a high-side P-channel MOSFET. A low output driver impedance of 2Ω allows the MIC2194 to drive large external MOSFETs to generate a wide range of output currents. The MIC2194 can achieve maximum duty cycles of 100%, which can be useful in low headroom applications.
The MIC2194 is available in an 8 pin SOIC package with a junction temperature range of –40°C to +125°C.
Features
• 2.9V to 14V input voltage range
• 400kHz oscillator frequency
• PWM current mode control
• 2Ω output drivers
• 100% maximum duty cycle
• 0.5µA micro-power shutdown
• Programmable UVLO
• Front edge blanking
• Cycle-by-cycle current limiting
• Frequency foldback short circuit protection
• 8-lead SOIC package
Applications
• Point of load power supplies
• Negative voltage buck-boost power supplies
• Distributed power systems
• Base stations
• Wireless modems
• ADSL line cards
• Servers
• Step down conversion in 3.3V, 5V, 12V systems
• 1-and 2-cell Li Ion battery operated equipment
T ypical Application
+3.3V
10µF
16V
V
IN
1µF
12V
47µF
20V (×2)
2.2nF
VIN EN/
UVLO VDD FB
2k
COMP
0.012Ω
MIC2194BM
CS
OUTP
GND
Si4431A (×2)
5.2µH
B530
10k
3.32k
V
OUT
5V, 5A
220µF 10V (×2)
Adjustable Output Buck Converter
V
IN
10µF
16V
1µF
4.99k
10nF
0.040Ω
MIC2194BM
VIN EN/
UVLO VDD FB
COMP
CS
OUTP
GND
Si9803
22µH
B530
3.01k
22nF
220µF 10V (×2)
1k
V
OUT
–5V, 0.6A
Positive-to-Negative Buck-Boost Converter
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
October 2002 1 MIC2194
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MIC2194 Micrel
Ordering Information
Part Number Output Voltage Frequency Junction Temp. Range Package
MIC2194BM Adjustable 400KHz –40°C to +125°C 8-lead SOP
Pin Configuration
FB
EN/UVLO
CS
Pin Description
Pin Number Pin Name Pin Function
1 COMP Compensation (Output): Internal error amplifier output. Connect to a
2 FB Feedback (Input): The circuit regulates this pin to 1.245V. 3 EN/UVLO Enable/Undervoltage Lockout (Input): A low level on this pin will power down
4 CS The (–) input to the current limit comparator. A built-in offset of 110mV
5 VDD 3V internal linear-regulator output. VDD is also the supply voltage bus for the
6 GND Ground. 7 OUTP High current drive for the synchronous N-channel MOSFET. Voltage swing
8 VIN Input voltage to the circuit. Also the high side input to the current sense
1COMP 2 3 4
8 VIN
OUTP
7
GND
6
VDD
5
8 Lead SOIC (M)
capacitor or series RC network to compensate the regulator’s control loop.
the device, reducing the quiescent current to under 0.5µA. This pin has two separate thresholds, below 1.5V the output switching is disabled, and below
0.9V the device is forced into a complete micropower shutdown. The 1.5V threshold functions as an accurate undervoltage lockout (UVLO) with hysteresis.
between VIN and CSL in conjunction with the current sense resistor sets the current limit threshold level. This is also the (–) input to the current amplifier.
chip. Bypass to GND with 1µF.
is from ground to VIN. On-resistance is typically 3Ω @ 5VIN.
amplifier supplies power to the gate drive circuit.
MIC2194 2 October 2002
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MIC2194 Micrel
Absolute Maximum Ratings (Note 1)
Supply Voltage (V
Digital Supply Voltage (VDD) ...........................................7V
Enable Pin Voltage (VEN)............................. –0.3V to +15V
Comp Pin Voltage (V
) .....................................................15V
IN
)............................ –0.3V to +3V
COMP
Operating Ratings (Note 2)
Supply Voltage (V
Junction Temperature ....................... –40°C ≤ TJ ≤ +125°C
Package Thermal Resistance
θJA 8-lead SOP .................................................140°C/W
) .................................... +2.9V to +14V
IN
Feedback Pin Voltage (VFB) .......................... –0.3V to +3V
Current Sense Voltage (VIN –VCS)................. –0.3V to +1V
Power Dissipation (PD) ..................... 285mW @ TA = 85°C
Ambient Storage Temp ............................ –65°C to +150°C
ESD Rating, Note 3 ...................................................... 2kV
Electrical Characteristics
VIN = 5V, V
Parameter Condition Min Typ Max Units Regulation
Feedback Voltage Reference (1%) 1.233 1.245 1.257 V
Feedback Bias Current 50 nA Output Voltage Line Regulation 5V ≤ VIN ≤ 9V 0.15 % / V Output Voltage Load Regulation 0mV < (VIN – VCS) < 75mV 0.9 % Output Voltage Total Regulation 5V ≤VIN ≤ 9V, 0mV < (VIN – VCS) < 75mV (±3%) 1.208 1.282 V
Input & VDD Supply
VIN Input Current (IQ) (excluding external MOSFET gate current) 1 2 mA Shutdown Current (ISD)V Digital Supply Voltage (VDD)I Digital Supply Load Regulation IL = 0 to 1mA 0.1 V Undervoltage Lockout VDD upper threshold (turn on threshold) 2.65 V UVLO Hysteresis 100 mV
Enable/UVLO
Enable Input Threshold 0.6 0.9 1.2 V UVLO Threshold 1.4 1.5 1.6 V Enable Input Current V
Current Limit
Current Limit Threshold Voltage VIN – VCS voltage to trip current limit 90 110 130 mV
Error Amplifier
Error Amplifier Gain 20 V/V
Current Amplifier
Current Amplifier Gain 3.0 V/V
Oscillator Section
Oscillator Frequency (fO) 360 400 440 kHz Maximum Duty Cycle VFB = 1.0V 100 % Minimum On Time VFB = 1.5V 165 ns Frequency Foldback Threshold Measured on FB 0.3 V Frequency Foldback Frequency 90 kHz
= 3.3V, TJ = 25°C, unless otherwise specified. Bold values indicate –40°C<TJ<+125°C.
OUT
(2%) 1.22 1.245 1.27 V
= 0V 0.5 5 µA
EN
= 0 2.82 3.0 3.18 V
L
EN/UVLO
= 5V 0.2 5 µA
October 2002 3 MIC2194
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MIC2194 Micrel
Parameter Condition Min Typ Max Units Gate Drivers
Rise/Fall Time CL = 3300pF 25 ns Output Driver Impedance Source, VIN = 12V 2 6 Ω
Sink, VIN = 12V 2 6 Ω Source, V Sink, VIN = 5V 3 7 Ω
= 5V 3 7 Ω
IN
Note 1. Absolute maximum ratings indicate limits beyond which damage to the component may occur. Electrical specifications do not apply when
Note 2. The device is not guaranteed to function outside its operating rating. Note 3. Devices are ESD sensitive, handling precautions required. Human body model, 1.5kΩ in series with 100pF.
operating the device outside of its operating ratings. The maximum allowable power dissipation is a function of the maximum junction temperature, T
, the junction-to-ambient thermal resistance, θJA, and the ambient temperature, TA.
J(Max)
MIC2194 4 October 2002
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MIC2194 Micrel
2.80
2.85
2.90
2.95
3.00
3.05
051015
V
DD
(V)
INPUT VOLTAGE (V)
VDDvs. Input Voltage
1.2430
1.2435
1.2440
1.2445
1.2450
1.2455
02468101214
REFERENCE VOLTAGE (V)
INPUT VOLTAGE (V)
Error Amp Reference Voltage
vs. Input Voltage
350
360
370
380
390
400
410
420
430
440
450
-50 -30 -10 10 30 50 70 90 110
SOFT START CURRENT (µA)
TEMPERATURE (°C)
Frequency Variation vs.
Temperature
VIN = 5V
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
02468101214
IMPEDANCE (Ω)
INPUT VOLTAGE (V)
OUTP Drive Impedance
vs. Input Voltage
Sink (Ω)
Source(Ω)
T ypical Characteristics
Quiescent Current
vs. Input Voltage
4.5 4
3.5 3
2.5 2
1.5 1
0.5
QUIESCENT CURRENT (mA)
0
0 5 10 15
3.05 VIN = 3.3V
3.03
3.01
2.99
2.97
VIN =5V
(V)
2.95
DD
V
2.93
VIN = 12V
2.91
2.89
2.87
2.85
0 0.2 0.4 0.6 0.8 1 1.2
VDD LOAD CURRENT (mA)
Switching
INPUT VOLTAGE (V)
VDDvs. Load
Standby
Quiescent Current
vs. Temperature
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
QUIESCENT CURRENT (mA)
0
-40 -20 0 20 40 60 80 100120
TEMPERATURE (°C)
VDDvs. Temperature
3.50
3.40
3.30
3.20
3.10
3.00
2.90
VDD (V)
2.80
2.70
2.60
2.50
-40 -20 0 20 40 60 80 100120
TEMPERATURE (°C)
VIN = 5V
VIN = 5V
Error Amp Reference Voltage
1.3
1.29
1.28
1.27
1.26
1.25
1.24
1.23
1.22
1.21
REFERENCE VOLTAGE (V)
VIN = 5V
1.2
-40 -20 0 20 40 60 80 100 120
Overcurrent Threshold vs.
130 125 120 115 110
October 2002 5 MIC2194
105 100
THRESHOLD (mV)
95 90
02468101214
vs. Temperature
TEMPERATURE (°C)
Input Voltage
INPUT VOLTAGE (V)
Frequency Variation
vs. Input Voltage
0.5
0
-0.5
-1
-1.5
FREQUENCY VARIATION (%)
-2 02468101214
INPUT VOLTAGE (V)
Current Limit Threshold
120 115 110 105 100
CURRENT LIMIT (mV)
vs. Temperature
VIN = 5V
95 90 85 80
-40 -20 0 20 40 60 80 100120
TEMPERATURE (°C)
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MIC2194 Micrel
Functional Diagram
V
IN
C
DECOUP
VIN
C
IN
8
OVERCURRENT
VREF
1.245V
COMPARATOR
EN/UVLO 3
VDD 5
COMP
0.1V Threshold
BIAS
VDD
CURRENT
∑
100k
SENSE
AMP
gm = 0.0002 gain = 20
ON
fs/4
OSC
COMPARATOR
1
CONTROL
RESET
SLOPE
COMPENSATION
PWM
GAIN
3
VIN
ERROR
AMP
CSH9
R
SENSE
CSL4
OUTP7
V
REF
FB
2
Q1
D1
L1
V
OUT
C
OUT
fs/4
FREQUENCY
FOLDBACK
Figure 1. MIC2194 Block Diagram
Functional Characteristics
Controller Overview and Functional Description
The MIC2194 is a BiCMOS, switched-mode, step down (buck) converter controller. It uses a P-channel MOSFET, which allows the controller to operate at 100% duty cycle and eliminates the need for a high side drive bootstrap circuit. Current mode control is used to achieve superior transient line and load regulation. An internal corrective ramp provides slope compensation for stable operation above a 50% duty cycle. The controller is optimized for high efficiency, high performance DC-DC converter applications.
Figure 1 is a block diagram of the MIC2194 configured as a buck converter. At the beginning of the switching cycle, the
0.3V
GND6
OUTP pin pulls low and turns on the high-side P-channel MOSFET, Q1. Current flows from the input to the output through the current sense resistor, MOSFET and inductor. The current amplitude increases, controlled by the inductor. The voltage developed across the current sense resistor, R
, is amplified inside the MIC2194 and combined with
SENSE
MIC2194 6 October 2002
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MIC2194 Micrel
The MIC2194 controller is broken down into several func­tions.
• Control loop
• PWM operation
• Current mode control
• Current limit
• Reference, enable and UVLO
• MOSFET gate drive
• Oscillator
Control Loop
PWM Control Loop
The MIC2194 uses current mode control to regulate the output voltage. This dual control loop method (illustrated in Figure 2) senses the output voltage (outer loop) and the inductor current (inner loop). It uses inductor current and output voltage to determine the duty cycle of the buck converter. Sampling the inductor current effectively removes the inductor from the control loop, which simplifies compen­sation.
V
IN
Switching
Converter
Switch Driver
I
INDUCTOR
V
ERROR
t
ON
t
PER
V
ERROR
D = tON/t
PER
I
INDUCTOR
V
REF
V
OUT
Voltage
Divider
Figure 2. Current Mode Control Example
As shown in Figure 1, the inductor current is sensed by measuring the voltage across the resistor, R
SENSE
. A ramp is added to the amplified current sense signal to provide slope compensation, which is required to prevent unstable opera­tion at duty cycles greater than 50%.
A transconductance amplifier is used for the error amplifier, which compares an attenuated sample of the output voltage with a reference voltage. The output of the error amplifier is the compensation pin (COMP), which is compared to the current sense waveform in the PWM block. When the current signal becomes greater than the error signal, the comparator turns off the high side drive. The COMP pin provides access to the output of the error amplifier and allows the use of external components to stabilize the voltage loop.
Current Limit
in Figure 1.). The
SENSE
current sense resistor must be sized using the minimum current limit threshold. The external components must be designed to withstand the maximum current limit. The current sense resistor value is calculated by the equation below:
MIN CURRENT SENSE THRESHOLD
R
SENSE
___
=
I
_
OUT MAX
The maximum output current is:
MAX CURRENT SENSE THRESHOLD
I
OUT MAX
_
___
=
R
SENSE
The current sense pins VIN (pin 8) and CSL (pin 4) are noise sensitive due to the low signal level, high input impedance and input ripple voltage. The PCB traces should be short and routed close to each other. A 0.1µF capacitor across the pins will attenuate high frequency switching noise.
When the peak inductor current exceeds the current limit threshold, the overcurrent comparator turns off the high-side MOSFET for the remainder of the switching cycle, effectively decreasing the duty cycle. The output voltage drops as additional load current is pulled from the converter. When the voltage at the feedback pin (FB) reaches approximately 0.3V, the circuit enters frequency foldback mode and the oscillator frequency will drop to 1/4 of the switching frequency. This limits the maximum output power delivered to the load under a short circuit condition.
Reference, Enable and UVLO Circuits
The output drivers are enabled when the following conditions are satisfied:
• The VDD voltage (pin 5) is greater than its undervoltage threshold.
• The voltage on the enable pin (pin 3) is greater than the enable UVLO threshold.
The enable pin (pin 3) has two threshold levels, allowing the MIC2194 to shut down in a low current mode, or turn off output switching in standby mode. An enable pin voltage lower than the shutdown threshold turns off all the internal circuitry and places the MIC2194 in a micropower shutdown mode.
If the enable pin voltage is between the shutdown and standby thresholds, the internal bias, VDD and reference voltages are turned on. The output drivers are inhibited from switching. The OUTP pin is in a high state. Raising the enable voltage above the standby threshold enables the output driver. The standby threshold is specified in the electrical characteristics. A resistor divider can be used with the enable pin to prevent the power supply from turning on until a specified input voltage is reached. The circuit in Figure 3 shows how to connect the resistors.
October 2002 7 MIC2194
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MIC2194 Micrel
MIC2194
1.5V
Typical
V
IN
R1
R2
EN/UVLO (3)
140mV
Hysteresis
(typical)
Bias
Circuitry
Figure 3. UVLO Circuitry
The line voltage turn on trip point is:
VV
INPUT ENABLE
_
=×
THRESHOLD
R
RR
+212
where:
V
THRESHOLD
is the voltage level of the internal
comparator reference, typically 1.5V. The input voltage hysteresis is equal to:
RR
VV
INPUT HYST HYST_
=×
+12
R
2
where:
V
is the internal comparator hysteresis level,
HYST
typically 140mV.
V
INPUT_HYST
is the hysteresis at the input voltage
The MIC2194 will be disabled when the input voltage drops back down to:
V
INPUT_OFF
V
INPUT_ENABLE
(V
THRESHOLD
=
– V
– V
INPUT_HYST
×
)
HYST
=
2
+RRR
12
• When the VDD voltage drops below its undervoltage lockout level.
• When the enable pin drops below the its enable threshold
MOSFET Gate Drive
The MIC2194 is designed to drive a high-side P-channel MOSFET. The source pin of the P-channel MOSFET is connected to the input of the power supply. It is turned on when OUTP pulls the gate of the MOSFET low. The advan­tage of using a P-channel MOSFET is that it does not require a bootstrap circuit to boost the gate voltage higher than the input, as would be required for an N-channel MOSFET. The VIN pin (pin 8) supplies the drive voltage to the gate drive pin, OUTP.
MOSFET Selection
The P-channel MOSFET must have a VGS threshold voltage equal to or lower than the input voltage when used in a buck converter topology. There is a limit to the maximum gate charge the MIC2194 will drive. MOSFETs with high gate charge will have slower turn-on and turn-off times. Slower transition times will cause higher power dissipation in the MOSFET due to higher switching transition losses.
The MOSFET gate charge is also limited by power dissipation in the MIC2194. The power dissipated by the gate drive circuitry is calculated below:
P
GATE_DRIVE
where: Q
GATE
= Q
GATE
× VIN × f
S
is the total gate charge of both the N- and P-
channel MOSFETs.
fS is the switching frequency VIN is the gate drive voltage
The graph in Figure 4 shows the total gate charge that can be driven by the MIC2194 over the input voltage range, for different values of switching frequency.
Max Gate Charge
250
200
150
100
50
MAMIMUM GATE CHARGE (nC)
0
0 5 10 15
INPUT VOLTAGE (V)
Figure 4. MIC2194 VIN vs Max. Gate Charge
Oscillator
A frequency foldback mode is enabled if the voltage on the feedback pin (pin 2) is less than 0.3V. In frequency foldback, the oscillator frequency is reduced by approximately a factor of 4. Frequency foldback is used to limit the energy delivered to the output during a short circuit fault condition.
Voltage Setting Components
The MIC2194 requires two resistors to set the output voltage as shown in Figure 5.
MIC2194 8 October 2002
Page 9
MIC2194 Micrel
V
MIC2194
Voltage
Amplifier
V
1.245V
Pin 2
REF
OUT
R1
R2
Figure 5
The output voltage is determined by:
R
VV
OUT
Where: V
=×+1
REF
for the MIC2194 is typically 1.245V.
REF
1
R
2
Lower values of R1 are preferred to prevent noise from appearing on the FB pin. A typically recommended value is 10kΩ. If R1 is too small in value it will decrease the efficiency of the power supply, especially at low output loads.
Once R1 is selected, R2 can be calculated with the following formula:
VR
×
R
REF
21=
VV
–
OUT
REF
Efficiency Considerations
Efficiency is the ratio of output power to input power. The difference is dissipated as heat in the buck converter. Under light output load, the significant contributors are:
• The VIN supply current, which includes the current required to switch the external MOSFET.
• Core losses in the output inductor.
To maximize efficiency at light loads:
• Use a low gate charge MOSFET or use the smallest MOSFET, which is still adequate for maximum output current.
• Use a ferrite material for the inductor core, which has less core loss than an MPP or iron power core.
Under heavy output loads the significant contributors to power loss are (in approximate order of magnitude):
• Resistive on time losses in the MOSFET
• Switching transition losses in the MOSFET
• Inductor resistive losses
• Current sense resistor losses
• Input capacitor resistive losses (due to the capacitors
ESR)
To minimize power loss under heavy loads:
• Use low on-resistance MOSFETs. Use low threshold logic level MOSFETs when the input voltage is below 5V. Multiplying the gate charge by the on-resistance gives a figure of merit, providing a good balance between low load and high load efficiency.
• Slow transition times and oscillations on the voltage and current waveforms dissipate more power during the turn on and turn off of the MOSFET. A clean layout will minimize parasitic inductance and capaci­tance in the gate drive and high current paths. This will allow the fastest transition times and waveforms without oscillations. Low gate charge MOSFETs will transition faster than those with higher gate charge requirements.
• For the same size inductor, a lower value will have fewer turns and therefore, lower winding resistance. However, using too small of a value will require more output capacitors to filter the output ripple, which will force a smaller bandwidth, slower transient response and possible instability under certain conditions.
• Lowering the current sense resistor value will de­crease the power dissipated in the resistor. However, it will also increase the overcurrent limit and will require larger MOSFETs and inductor components.
• Use low ESR input capacitors to minimize the power dissipated in the capacitors ESR.
October 2002 9 MIC2194
Page 10
MIC2194 Micrel
Package Information
0.026 (0.65) MAX)
PIN 1
0.157 (3.99)
0.150 (3.81)
0.050 (1.27)
0.064 (1.63)
0.045 (1.14)
TYP
0.197 (5.0)
0.189 (4.8)
DIMENSIONS:
INCHES (MM)
0.020 (0.51)
0.013 (0.33)
0.0098 (0.249)
0.0040 (0.102)
0°–8°
SEATING
PLANE
8-Pin SOP (M)
45°
0.050 (1.27)
0.016 (0.40)
0.244 (6.20)
0.228 (5.79)
0.010 (0.25)
0.007 (0.18)
MICREL, INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel, Inc.
© 2002 Micrel, Incorporated
MIC2194 10 October 2002
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