MIC24085
18V, 3A, 1MHz, High-Performance,
Integrated FET Buck Regulator
in Tiny 3mm × 3mm QFN
Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
October 22
, 2013
Revision 1.1
General Description
Micrel’s MIC24085 is a constant-frequency, current-mode
PWM buck regulator with integrated switchers. The
MIC24085 is targeted for cost-sensitive and high-
performance applications.
The MIC24085 operates over a supply range of 4.5V to
18V at a fixed 1MHz switching frequency and can be used
to provide up to 3A of output current. The output voltage is
adjustable down to 0.9V.
The MIC24085 provides the features of low standby
current at 10µA, device enable, input undervoltage lockout,
and output voltage monitoring. The part also provides fault
protections of overcurrent and overtemperature.
The MIC24085 is available in a 16-pin 3mm × 3mm QFN
package with a junction operating range from –40°C to
+125°C.
Datasheets and support documentation are available on
Micrel’s web site at: www.micrel.com.
Features
• Input voltage range 4.5V to 18V
• 3A output current
• 1MHz switching frequency
• 0.9V reference voltage with ±1.5% accuracy
• Fixed output voltages are available at:
1.5V/1.8V/2.5V/3.3V/5V
• Peak current-mode PWM with internal compensation
• PFM mode for light load efficiency
• 10µA typical shutdown current
• 4ms internal soft start
• Cycle-by-cycle current limit with frequency foldback
• Output overvoltage protection
• Enable input/power-good (PG) output
• Thermal-shutdown protection
• –40°C to +125°C junction temperature range
• Available in 16-pin 3mm × 3mm QFN package
Applications
• Camera, set-top box, gateways, and routers
• Networking/telecom infrastructure
• Printers, scanners, graphic cards, and video cards
Typical Applications
Micrel, Inc.
MIC24085
October 22
, 2013 2 Revision 1.1
Ordering Information
Part Number Output Voltage Junction Temperature Range Package Lead Finish
MIC24085AYML ADJ
–40°C to +125°C 16-Pin 3mm x 3mm QFN Pb-Free
MIC24085BYML 1.5V
MIC24085CYML 1.8V
MIC24085DYML 2.5V
MIC24085EYML 3.3V
MIC24085FYML 5V
Pin Configuration
16-Pin 3mm × 3mm QFN (ML)
(Top View)
Pin Description
Pin Number
Pin Name
Pin Function
1, 12, 13, 16 SW Switch Node (Output): Power stage output. Connect an inductor to this pin.
2, 3, 11 VCC Supply Voltage (for control circuit and power stage): A ceramic input capacitor between VCC and
the power ground (PGND) is required.
4 GND Analog Ground: GND is the ground path of the control circuitry.
5 FB
Feedback (Input): Input to the error amplifier of the PWM controller. The FB pin is regulated to
0.9V. A resistor divider connecting the feedback to the output is used to adjust the desired output
voltage.
6, 7 NC No connection.
8 PG Power Good (Output): Open drain output, which needs a pull-up. The PG pin turns logic high when
the converter output voltage is regulated in the range from 93% to 103% of the setting.
9 EN Enable (Input): An accurate Enable with hysteresis is provided. High = enable, Low = shutdown.
Enable has an internal pull-up to ensure a stable state if this pin is not connected.
10 BST Boost (Output): Bootstrapped voltage to the high-side N-channel MOSFET. A boost capacitor of
0.1μF is connected between the BST pin and the SW pin.
14,15 PGND Power Ground: PGND is the ground path for the power stage.
EP ePAD Exposed Pad. It must be connected to PGND on the PCB.
Micrel, Inc.
MIC24085
October 22
, 2013 3 Revision 1.1
Absolute Maximum Ratings(1)
VVCC to PGND ................................................ −0.3V to +20V
VSW to PGND ................................................. −0.3V to +20V
VEN, VFB, VPG, to GND ..................................... −0.3V to + 6V
VBST to VSW ..................................................... −0.3V to + 6V
VBST to PGND ................................................ −0.3V to +26V
PGND to GND ............................................. −0.3V to + 0.3V
Junction Temperature .............................................. +150°C
Storage Temperature (TS) ......................... −65°C to +150°C
Lead Temperature (soldering, 10sec) ........................ 260°C
ESD Rating(3) .................................................................. 2kV
Operating Ratings(2)
Supply Voltage (VVCC) ....................................... 4.5V to 18V
Output Current ................................................................ 3A
Junction Temperature (TJ) ........................ −40°C to +125°C
Junction Thermal Resistance
MLF-16 (θJA) ..................................................... 68°C/W
MLF-16 (θJC) .................................................... 7.5°C/W
Electrical Characteristics(4)
VVCC = 12V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter Condition Min. Typ. Max. Units
Power Supply Input
VCC Input Voltage Range 4.5 18 V
Quiescent Supply Current No Switching, VFB = 1.5V 2.3 mA
Shutdown Supply Current VEN = 0V 5 10 µA
VCC UVLO Threshold 4.3 4.5 V
VCC UVLO Hysteresis 650 mV
Reference (MIC24085AYML)
Feedback Reference Voltage -40°C ≤ TJ ≤ +125°C (±1.5%) 0.886 0.9 0.914 V
FB Bias Current VFB = 0.6V 50 500 nA
Soft-Start Time 4 ms
Load Regulation IOUT = 100mA to 3A 0.5 %
Line Regulation VVCC = 4.5V to 18V 0.5 %
Enable Control
EN Logic Level High 2.2 V
EN Logic Level Low
0.8
V
EN Hysteresis 480 mV
EN Bias Current VEN = 5V 3.5 µA
VEN = 0V -1.5
PWM
Switching Frequency VFB = 0.9V 0.9 1.0 1.1 MHz
Foldback Switching Frequency VFB < 60% × 0.9V 250 kHz
Maximum Duty Cycle VFB = 0V, 1MHz 97 %
Minimum Duty Cycle VFB = 1V 0 %
Notes:
1. Exceeding the absolute maximum ratings may damage the device.
2. The device is not guaranteed to function outside its operating ratings.
3. Devices are ESD sensitive. Handling precautions are recommended. Human body model, 1.5kΩ in series with 100pF.
4. Specification for packaged product only.
Micrel, Inc.
MIC24085
October 22
, 2013 4 Revision 1.1
Electrical Characteristics(4) (Continued)
VVCC = 12V; TA = 25°C, unless noted. Bold values indicate −40°C ≤ TJ ≤ +125°C.
Parameter Condition Min. Typ. Max. Units
Internal MOSFET
High Side On-Resistance ISW = 1A 135 mΩ
Low Side On-Resistance ISW = 1A 90 mΩ
Overcurrent Protection
Current-Limit Threshold VFB = 0.9V 3.4 3.8 4.4 A
Power Good
Power Good Threshold Voltage Sweep VFB from Low-to-High 91 93 95 %VOUT
Power Good Hysteresis Sweep VFB from High-to-Low 1.5 %VOUT
Power Good Low Voltage VFB < 90% x VNOM, IPG = -0.3mA 50 100 mV
Power Good Recovery Hold Time PG recovery after power
restoration 9 11 12 ms
Thermal Protection
Overtemperature Shutdown TJ Rising 170 °C
Overtemperature Shutdown Hysteresis 10 °C
Micrel, Inc.
MIC24085
October 22
, 2013 5 Revision 1.1
Typical Characteristics
TJ = −40°C to +125°C, VCC = 12V (unless otherwise noted).
Micrel, Inc.
MIC24085
October 22
, 2013 6 Revision 1.1
Typical Characteristics (Continued)
TJ = −40°C to +125°C, VCC = 12V (unless otherwise noted).
Micrel, Inc.
MIC24085
October 22
, 2013 7 Revision 1.1
Functional Characteristics
TJ = −40°C to +125°C, VCC = 12V (unless otherwise noted).
Micrel, Inc.
MIC24085
October 22
, 2013 8 Revision 1.1
Functional Diagram
Micrel, Inc.
MIC24085
October 22
, 2013 9 Revision 1.1
Functional Description
Micrel’s MIC24085 is an 18V, 3A, 1MHz buck regulator
with integrated MOSFETSs for low-cost high-
performance applications. The MIC24085 provides basic
features including enable, power good, internal soft-start,
current-limit, and overtemperature protections.
VCC Supply Voltage and UVLO
The MIC24085 is held in the off state until VCC reaches
the UVLO threshold, which has a 650mV hysteresis. A
ceramic capacitor from VCC to PGND is recommended
for clean operation.
Enable and Soft-Start
When the MIC24085 is enabled, the reference voltage is
controlled to ramp-up from 0V to 0.9V in approximately
4ms. During the soft-start, the current limit is enabled to
protect the device in case of a short circuit present at the
output.
Peak Current-Mode PWM
To achieve fast transient response, the peak current-
mode PWM control is employed in the MIC24085. As
illustrated in the Functional Diagram, the high-side
MOSFET current (ISW) is sensed and summed with a
slope-compensating ramp. Then it is compared with the
error amplifier output to generate a PWM signal.
The loop compensation is designed internally in the
MIC24085 to ensure loop stability, even with a low-ESR
capacitor at the output, such as a ceramic output
capacitor.
PFM Mode at Light Load
The MIC24085 runs in asynchronous buck mode at light
load, where the bottom MOSFET is turned off when the
inductor current drops to zero. This scheme prevents the
output energy from being pumped back to the input, and
improves the over-all efficiency.
Current Limit and Frequency Foldback
In the MIC24085 ISW is also used to determine over-
current conditions. As the peak inductor current is limited,
the output voltage of the buck converter drops as the load
current increases. When the output voltage reaches 60%
of regulated voltage (or feedback voltage reaches 60% of
0.9V), the switching frequency is forced to fold back, from
1MHz to 250kHz. This current-limit scheme reduces the
average inductor current (even with the same peak
inductor current), and improves the power consumption
at the overload conditions.
The current limit is always active when the regulator is
enabled. Once the overcurrent condition is removed, the
device runs to a soft-start and returns to a normal
operation.
Power Good (PG)
Power good (PG) is an open drain output pin. A 10kΩ
resistor to VOUT is required for pull-up. The PG pin turns
low if the output voltage is below 93% of the reference
voltage (0.9V). After VOUT fault and recovery, PG is held
low for a typical duration of 11ms. See the Electrical
Characteristics table for additional information.
Channel 1: VOUT, Channel 2: PG, Hold Time: 11ms
Output Voltage Protection
The MIC24085 turns both high-side and low-side
MOSFETs off to shut down the buck regulator output
voltage when the output voltage exceeds 103% of the
regulated voltage. After the output voltage drops back
below the threshold, the MIC24085 runs to a soft-start.
Thermal Shutdown
The MIC24085 turns both high-side and low-side
MOSFETs off to shut down the buck regulator output
voltage when the internal temperature is over the setting
threshold at around 170°C. After the temperature drops
back below the threshold, the MIC24085 runs to a soft-
start.
Micrel, Inc.
MIC24085
October 22
, 2013 10 Revision 1.1
Transient Response and Control Loop Bandwidth
MIC24085 with adjustable output voltage has been
designed to work under a wide range of input and output
voltages, supporting different values and types of output
capacitance. By design, the MIC24085 has a lower
control loop bandwidth. For designs with aggressive load
step response requirements, using a fixed-output voltage
version with a larger output capacitance is
recommended.
Adjustable Output Voltage: VIN = 12V, VOUT = 5V,
Load Step = 0.5A to 2.5A at 50mA/µs with
1x470µF and 1x100µF output capacitance.
Channel 1: VOUT, Channel 2: PG, Channel 3: Load Step
Micrel, Inc.
MIC24085
October 22
, 2013 11 Revision 1.1
Evaluation Board Schematic
Bill of Materials
Item Part Number Manufacturer Description Qty.
C1 GRM31CR61E106KA12 Murata
(5)
10µF/25V Ceramic Capacitor, X5R, Size 1206 1
C2, C5 GRM188R71H104KA93D Murata 0.1µF/50V Ceramic Capacitor, X7R, Size 0603 2
C3 GRM31CF51A226ZE01L Murata 22µF/10V Ceramic Capacitor, X5R, Size 1206 1
C4 (Open) Murata 22µF/10V Ceramic Capacitor, X5R, Size 1206 1
D1 (Open) SK33A Diodes, Inc.(6) 40V Small Signal Schottky Diode, SOD123 1
L1 CIP0530LR3R3 ABC Taiwan Electronics
Corporation(7) 3.3µH Inductor, 5A Sat. Current 1
R1, R4 CRCW06033K32FKEA Vishay Dale(8) 3.32kΩ Resistor, Size 0603, 1% 1
R2, R8 CRCW060310K0FKEA Vishay Dale 10kΩ Resistor, Size 0603, 1% 2
R3 CRCW06034K99FKEA Vishay Dale 4.99kΩ Resistor, Size 0603, 1% 1
R5 CRCW06031K87FKEA Vishay Dale 1.87kΩ Resistor, Size 0603, 1% 1
R6 CRCW06031K24FKEA Vishay Dale 1.24kΩ Resistor, Size 0603, 1% 1
R7 CRCW0603732R0FKEA Vishay Dale 732Ω Resistor, Size 0603, 1% 1
U1 MIC24085AYML Micrel Inc.(9) 18V, 3A, 1MHz Tiny Buck Regulator 1
Notes:
5. Murata: www.murata.com.
6. Diodes, Inc.: www.diodes.com.
7. ABC Taiwan Electronics Corporation: www.atec-group.com.
8. Vishay: www.vishay.com.
9. Micrel, Inc.: www.micrel.com.
Micrel, Inc.
MIC24085
October 22
, 2013 12 Revision 1.1
PCB Layout Recommendations
Recommended Component Placement at Top Layer
Micrel, Inc.
MIC24085
October 22
, 2013 13 Revision 1.1
PCB Layout Recommendations (Continued)
Top Layer
Mid-Layer 1 (Ground Plane)
Micrel, Inc.
MIC24085
October 22
, 2013 14 Revision 1.1
PCB Layout Recommendations (Continued)
Mid-Layer 2 (Ground Plane)
Bottom Layer (Ground Plane)
Micrel, Inc.
MIC24085
October 22
, 2013 16 Revision 1.1
MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA
TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com
Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This
information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry,
specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual
property rights is granted by this document. Except as provided in Micrel’s terms and conditions of sale for such products, Micrel assumes no liability
whatsoever, and Micrel disclaims any express or implied warranty relating to the sale and/or use of Micrel products including liability or warranties
relating to fitness for a particular purpose, merchantability, or infringement of any patent, copyright or other intellectual property right.
Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product
can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical
implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A
Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully
indemnify Micrel for any damages resulting from such use or sale.
© 2013 Micrel, Incorporated.
