MIC MIC3838 Flexible push-pull pwm controller Datasheet

MIC3838/3839
Micrel
MIC3838/3839
Flexible Push-Pull PWM Controller
General Description
Features
The MIC3838 and MIC3839 are a family of complementary
output push-pull PWM control ICs that feature high speed and
low power consumption. The MIC3838/9 are ideal for telecom
level (36V to 75V) isolated step down dc/dc conversion
applications where high output current, small size, and high
efficiency are required
The MIC3838/9 are designed for high flexibility with minimum
pin-count. The devices are easily configurable for either
voltage-mode or current-mode control. Additionally, the
MIC3838/9 can easily implement a volt-second clamp that
automatically limits the duty cycle during input transients,
allowing designers to use the smallest possible transformers
and power components. A 3V reference output is also available that eliminates the need for an external reference.
The dual-ended push-pull architecture of the MIC3838/9
allows more efficient utilization of the transformer than singleended topologies, allowing smaller size dc/dc solutions.
Additionally, the out-of-phase push-pull topology allows a
higher effective duty cycle, reducing input and output ripple
as well as stress on the external components. The dead-time
between the two outputs is adjustable between 60ns to
200ns, limiting the duty cycle of each output stage to less than
50%.
The MIC3838 has a turn-on threshold of 12.5V whereas the
MIC3839 has a lower turn-on threshold of 4.3V. Both devices
are available in a small size MSOP-10 package with an
operating range of –40°C to +85°C.
Data sheets and support documentation can be found on
Micrel’s web site at www.micrel.com.
•
•
•
•
•
•
•
•
•
Dual output drive stages in push-pull configuration
Configurable for current-mode or voltage-mode control
Easily implements volt-second clamp
Leading edge current-sense blanking
3V reference output available
130µA typical start-up current
1mA typical run current
Operation to 1MHz
On-chip error amplifier with 4MHz gain bandwidth
product
• Internal soft start
• On-chip VDD clamping
• Output drive stages capable of 500mA peak source
current, 1A peak sink current
Applications
• High efficiency “brick” power supply modules
• Half bridge converters
• Full bridge converters
• Push-pull converters
• Voltage-fed push-pull converters
• Telecom equipment and power supplies
• Industrial power supplies
• 42V automotive power supplies
• Base stations
• Networking power supplies
Typical Application
VIN
36V to 75V
VOUT
12V 100W
Start-Up
Circuitry
MIC3838x-x
VDD
RC
OUTA
GND
OUTB
COMP
FB
RAMP
ILIM
Driver
VREF
Feed Forward
Ramp/
Volt Sec Clamp
Reference
& Isolation
Voltage-Mode Half-Bridge Converter CIrcuit
Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com
April 2005
1
MIC3838/3839
MIC3838/3839
Micrel
Ordering Information
Part Number
Lead-Free
Turn On
Threshold
Turn Off
Threshold
Temperature Range
Package
MIC3838BMM
MIC3838YMM
12.5V
8.3V
–40°C to +85°C
10-Pin MSOP
MIC3839BMM
MIC3839YMM
4.3V
4.1V
–40°C to +85°C
10-Pin MSOP
Standard
Pin Configuration
COMP 1
10 VREF
FB 2
9 VDD
ILIM 3
8 OUTA
RAMP 4
7 OUTB
RC 5
6 GND
MSOP-10 (MM)
Pin Description
Pin Number
Pin Name
Pin Function
1
COMP
2
FB
3
ILIM
4
RAMP
Input to the PWM comparator. Sawtooth ramp for PWM control. Allows for
either current-mode or voltage-mode control. An internal MOSFET discharges the current sense filter capacitor.
5
RC
The oscillator programming pin. Only two components are required to
program the oscillator, a resistor (tied between VDD and RC), and a capacitor (tied between RC and GND). The approximate oscillator frequency is
COMP is the output of the error amplifier and the input of the PWM
comparator. The error amplifier in the MIC3838 is a true low-output impedance, 4MHz operational amplifier. As such, the COMP pin can both source
and sink current. However, the error amplifier is internally current limited, so
that zero duty cycle can be externally forced by pulling COMP to GND.
The MIC3838 family features built-in full cycle soft start. Soft start is implemented as a clamp on the maximum COMP voltage.
The inverting input to the error amplifier.
The input to the peak current, and overcurrent comparators. The
overcurrent comparator is only intended for fault sensing. Exceeding the
overcurrent threshold will cause a soft start cycle. An internal MOSFET
discharges the current sense filter capacitor to improve dynamic performance of the power converter.
determined by the simple formula:
FOSCILLATOR =
1.41
R×C
The recommended range of timing resistors is between 7kΩ and 200kΩ and
range of timing capacitors is between 100pF and 1000pF. Timing resistors
less than 7kΩ should be avoided. For best performance, keep the leads
between components as short as possible. Separate ground and VDD
traces to the external timing network are encouraged.
6
MIC3838/3839
GND
Ground. Return path for signal and gate drive functions.
2
April 2005
MIC3838/3839
Micrel
Pin Description
Pin Number
Pin Name
7, 8
OUTB, OUTA
Alternating high current output stages. Both stages are capable of driving
the gate of a power MOSFET. Each stage is capable of 500mA peak source
current, and 1A peak sink current.
The output stages switch at half the oscillator frequency, in a push/pull
configuration. When the voltage on the RC pin is rising, one of the two
outputs is high, but during fall time, both outputs are off. This “dead time”
between the two outputs, along with a slower output rise time than fall time,
insures that the two outputs can not be on at the same time. This dead time
is typically 60ns to 200ns and depends upon the values of the timing
capacitor and resistor.
The high-current output drivers consist of MOSFET output devices, which
switch from VDD to GND. Each output stage also provides a very low
impedance to overshoot and undershoot. This means that in many cases,
external Schottky clamp diodes are not required.
9
VDD
The power input connection for this device. Total VDD current is the sum of
quiescent VDD current and the average gate drive (OUT) current. Knowing
the operating frequency and the MOSFET gate charge (Qg), average OUT
current can be calculated from IOUT = Qg • F, where Qg is the total gate
change of all MOSFETs (OUTA and OUTB) and F is oscillator switching
frequency. To prevent noise problems, bypass VDD to GND with a ceramic
capacitor as close to the chip as possible. A 1µF decoupling capacitor is
recommended.
10
VREF
Internal 3V supply. Will source 1mA maximum.
April 2005
Pin Function
3
MIC3838/3839
MIC3838/3839
Micrel
Absolute Maximum Rating (Note 1)
Operating Ratings (Note 2)
Supply Voltage (IDD ≤10mA) ...................................... +15V
Supply Current ........................................................... 20mA
OUTA/OUTB Source Current (peak) ...................... –0.5A
OUTA/OUTB Sink Current (peak) ............................ 1.0A
COMP Pin .................................................................... VDD
Analog Inputs (FB, ILIM, RAMP) ......... –0.3V to VDD +0.3V
NOT TO EXCEED 6V
Junction Temperature .............................. –55°C to +150°C
Storage Temperature (TS) ....................... –65°C to +150°C
Lead Temperature (soldering, 10 sec.) ................... +300°C
ESD Rating, Note 3 ...................................................... 2kV
VDD Input Voltage (VDD) .......................................... Note 4
Oscillator Frequency (fOSC) ....................... 10kHz to 1MHz
Ambient Temperature (TA) ......................... –40°C to +85°C
Package Thermal Resistance
MSOP-10 (θJA) .................................................. 115°C/W
Electrical Characteristics (Note 5)
TA = TJ = –40°C to +85°C, VDD=10V, Note 10,1µF capacitor from VDD to GND, R=22kΩ, C=330pF.
Parameter
Condition
Min
Typ
Max
Units
Output voltage
IOUT = 0mA
2.85
3.0
3.15
V
Line Regulation
MIC3838 9V ≤ VDD ≤12V
MIC3839 5V ≤ VDD ≤12V
IOUT = 1mA
2
10
mV
14
30
mV
180
200
220
kHz
Note 6
0.44
0.5
0.56
V/VDD
COMP = 2V
1.95
2
2.05
V
1
µA
Internal Reference Section
Load Regulation
Oscillator Section
Oscillator Frequency
Oscillator Amplitude/VDD
Error Amp Section
Input Voltage
Input Bias Current
–1
Open Loop Voltage Gain
(Guaranteed by design)
60
80
dB
COMP Sink Current
FB = 2.2V, COMP = 1V
0.3
2.5
mA
COMP Source Current
FB = 1.3V, COMP = 3V, Note 7
–0.15
–0.5
mA
COMP PM Clamp Voltage
VFB = 0V
3.1
3.6
4.0
V
Maximum Duty Cycle
Measured at OUTA or OUTB
48
49
50
%
Minimum Duty Cycle
COMP = 0V
0
%
PWM Section
Current Sense Section
Gain
Guaranteed by design, Note 8
1.9
2.2
2.5
V/V
ILIM Maximum Input Signal
Note 9
0.45
0.5
0.55
V
ILIM to Output Delay
COMP = 3V, ILIM from 0mV to 600mV
70
200
ns
Ramp or ILIM Source Current
Ramp or ILIM Sink Current
–200
Ramp = ILIM = 0.5V, RC = 5.5V Note 10
ILIM Over Current Threshold
COMP to Ramp Offset
Ramp = ILIM = 0V
nA
5
10
mA
0.7
0.75
0.8
V
0.35
0.8
1.2
V
Output Section
OUT Low Level
I = 100mA
0.5
1
V
OUT High Level
I = –50mA, VDD – OUT
0.5
1
V
Rise Time
CL = 1nF
25
60
ns
Fall Time
CL = 1nF
25
60
ns
MIC3838/3839
4
April 2005
MIC3838/3839
Micrel
Parameter
Condition
Min
Typ
Max
Units
MIC3838, Note 11
11.5
12.5
13.5
V
MIC3839
4.1
4.3
4.5
V
Minimum Operating Voltage
MIC3838
7.6
8.3
9
V
After Start
MIC3839
3.9
4.1
4.3
V
Hysteresis
MIC3838
3.5
4.2
5.1
V
MIC3839
0.1
0.2
0.3
V
FB = 1.8V, Rise from 0.5V to 3V
2.5
20
ms
Startup Current
VDD < Start Threshold (MIC3839)
130
260
µA
Operating Supply Current
FB = 0V, Ramp = ILIM = 0V, Notes 11, 12
1.5
2
mA
VDD Zener Shunt Voltage
IDD = 10mA, Note 13
14
15
V
Undervoltage Lockout Section
Start Threshold
Soft Start Section
COMP Rise Time
Overall Section
13
Note 1.
Exceeding the absolute maximum rating may damage the device.
Note 2.
The device is not guaranteed to function outside its operating rating.
Note 3.
Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF.
Note 4.
Maximum operating voltage is equal to the VDD (zener) shunt voltage. When operating at or near the shunt voltage, care must be taken to limit
the VDD pin current less than the 20mA VDD maximum current rating.
Note 5.
Specification for packaged product only.
Note 6.
Measured at RC.
Note 7.
COMP pin is internally clamped to 3.65V(typ.). COMP pin source current is measured at VCOMP = 3.0V to avoid interferring with clamp.
Minimum source current is higher as VCOMP approaches VCLAMP.
Note 8.
Gain is defined by A =
Note 9.
Parameter measured at trip point of latch with FB at 0V
∆VCOMP
∆VCS
, 0 ≤ VCS ≤ 0.4V.
Note 10. The internal current sink on the Ramp and ILIM pin are designed to discharge an external filter capacitor. It is not intended to be a DC sink
path. Internal discharge FET should be able to discharge the Volt-Sec clamp and feed-forward circuits in the figure below within 50ns.
Note 11. For MIC3838, set VDD above the start threshold before setting at 10V.
Note 12. Does not include current in the external oscillator network.
Note 13. Start threshold and Zener Shunt threshold track one another.
April 2005
5
MIC3838/3839
MIC3838/3839
Micrel
Typical Characteristics
MIC3839
IDD Current
MIC3838
IDD Current
10.0
16.0
7.0
6.0
5.0
4.0
3.0
2.0
IDD CURRENT (mA)
8.0
12.0
10.0
8.0
6.0
4.0
1.240
VIN = 10V
1.230
MIC3839 IDD Current
vs. Temperature
C = 100pF
FREQUENCY (kHz)
VIN = 10V
1.200
VIN = 5V
1.100
140
120
80
RC Pin Capacitance
vs. Deadtime
200
VDD = 10V
175
C = 220pF
C = 270pF
C = 330pF
C = 470pF
C = 680pF
100k
100
TEMPERATURE (°C)
Oscillator Frequency
vs. RC Values
1M
1.300
1.220
6 8 10 12 14
VDD (V)
60
4
40
2
0
0
20
0
6 8 10 12 14
VDD (V)
-20
4
-40
2
DEADTIME (ns)
0
1.400
IDD CURRENT (mA)
1.250
2.0
1.0
0
1.260
14.0
IDD CURRENT (mA)
IDD CURRENT (mA)
9.0
MIC3838 IDD Current
vs. Temperature
150
125
100
1.500
MIC3839 Oscillator Variation
vs. VDD
2.04
0.4
0.35
0.25
0.1
140
120
100
TEMPERATURE (°C)
MIC3839 Error Amplifier
Reference vs. VDD
2.040
MIC3838 Error Amplifier
Reference vs. Temperature
2.030
1.98
VIN = 10V
2.020
2.010
6
1.990
140
18
120
16
100
10 12 14
VDD (V)
80
8
60
6
40
1.92
4
20
14
0
12
-20
2.000
1.94
-40
0
MIC3838/3839
80
60
40
20
-20
-1.500
0
-1.000
1.96
OPERATING
6
8 10
VDD (V)
900
-0.500
VREF (V)
VREF (V)
0.15
4
800
VIN = 10V
0.000
2.00
0.2
2
700
0.500
2.02
0.3
0.05
600
500
400
VIN = 5V
1.000
-40
140
120
80
100
-40
2.000
MIC3839 % Oscillator
Variation vs. Temperature
TEMPERATURE (°C)
RESISTANCE (kΩ)
OSCILLATOR (%)
60
-0.50
-1.00
200
180
160
140
120
80
100
40
0
20
60
84
0.00
40
85
0.50
0
86
VIN = 10V
20
87
1.00
-20
FREQUENCY VARIATION (%)
DEADTIME (ns)
88
MIC3838 Oscillator Variation
vs. Temperature
FREQUENCY VARIATION (%)
1.50
89
-0.05
-0.1
0
300
CAPACITANCE (pF)
RC Pin Resistance
vs. Deadtime
83
200k
1000
TEMPERATURE (°C)
50
0
50k
100k
150k
RESISTANCE (kΩ)
100
10k
0
140
120
80
100
60
40
0
20
-20
-40
1.000
200
75
C = 1000pF
TEMPERATURE (°C)
April 2005
MIC3838/3839
MIC3839 Error Amplifier
Reference vs. Temperature
VIN = 5V
MIC3839 Peak Current Limit
vs. VDD
0.540
0.55
VIN = 10V
2.020
2.010
VTHRESHOLD (V)
VREF (V)
2.030
MIC3838 Current Limit
Threshold vs. VDD
0.56
VTHRESHOLD (V)
2.040
Micrel
0.54
0.53
0.52
0.535
0.530
0.525
0.5
8
140
120
80
100
60
40
0
20
-40
2.000
-20
0.51
TEMPERATURE (°C)
0.545
MIC3838 Peak Current Limit
vs. Temperature
0.550
9
0.520
10 11 12 13 14 15 16
VDD (V)
0
2
4
6 8 10 12 14
VDD (V)
MIC3839 3V Reference
Voltage vs. VDD
MIC3839 Peak Current Limit
vs. Temperature
3.020
0.545
VIN = 10V
0.525
0.535
0.530
0.525
VIN = 5V
VIN = 10V
0.520
3.000
VREF (V)
0.535
0.530
3.010
0.540
VTHRESHOLD (V)
VTHRESHOLD (V)
0.540
2.980
0.515
0.510
0.520
2.990
2.970
0.505
MIC3839 3V Reference
vs. Temperature
2.950
-40 -20 0 20 40 60 80 100120140
TEMPERATURE (°C)
90
20
0
PHASE (°)
40
45
-20
-40
0
1E+7
2.960
135
60
1E+6
2.970
80
1E+5
2.980
180
100
1E+4
2.990
120
1E+3
VIN = 10V
3.060
3.050
3.040
3.030
VIN = 5V
3.020
3.010
3.000
2.990
VIN = 10V
2.980
2.970
2.960
2.950
-40 -20 0 20 40 60 80 100120140
TEMPERATURE (°C)
1E+2
VREF (V)
3.000
VREF (V)
3.010
MIC3838 Error Amplifier
Phase-Gain vs. Frequency
1E+1
MIC3838 3V Reference
vs. Temperature
2.960
4 5 6 7 8 9 10 11 12 13 14
VDD (V)
GAIN (dB)
3.020
0.500
-40 -20 0 20 40 60 80 100120140
TEMPERATURE (°C)
1E+0
0.515
-40 -20 0 20 40 60 80 100120140
TEMPERATURE (°C)
FREQUENCY (Hz)
MIC3839 3V Reference Voltage
vs. Current
3.02
3.01
VREF (V)
3.00
VDD = 4.3V
2.99
2.98
VDD = 10.0V
2.97
2.96
0
April 2005
0.5
1.0
1.5
REFERENCE CURRENT (mA)
7
MIC3838/3839
MIC3838/3839
Micrel
Functional Diagram
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Figure 1. MIC3838 Block Diagram
VIN = 36V to 75V
150k
13k
MIC3838
VCOMP
50k
4.5k
PWM
Comparator
330pF
0.5V
Peak Current
Comparator
0.75V
Over Current
Comparator
330pF
VMAX at Ramp Input = 1.5V
VMAX at ILIM Input = 0.5V
Oscillator
Figure 2. Volt Second Clamp and Voltage Feed Forward Circuit
VDD and Turn-on Sequence
The oscillator and output gate drive signals are disabled
when VDD is lower than the turn on threshold. Circuitry in the
output drivers eliminates glitching or random pulsing during
the start-up sequence. The oscillator is enabled when VDD is
applied and reaches the turn-on threshold. The VDD comparator also turns off the internal soft-start discharge FET,
slowly bringing up the COMP pin voltage.
The VDD pin is internally clamped. As VDD approaches this
clamp voltage, the VDD current will increase over the normal
current draw of the IC. IDD currents greater than 20mA may
cause excessive power dissipation in the MIC3838/9.
Functional Description
The MIC3838/9 is a high-speed power supply controller with
push-pull output drive capability. MIC3838 has a higher VDD
turn-on threshold and more hysteresis between VDD turn-on
and turn-off than the MIC3839. The outputs of the controller
operate in a push-pull fashion with a guaranteed dead time
between them. A block diagram of the MIC3838/9 controller
is shown in Figure 1.
MIC3838/3839
8
April 2005
MIC3838/3839
Micrel
Graphs of oscillator frequency and dead time vs component
values are shown in the Typical Characteristic section of this
specification. The recommended range of timing resistors
and capacitors is 7kΩ to 200kΩ and 100pF to 1000pF. To
minimize oscillator noise and insure a stable waveform the
following layout rules should be followed:
1. The higher impedance of capacitor values less
than 100pF may causes the oscillator circuit to
become more susceptible to noise. Parasitic pin
and etch trace capacitances become a larger
part of the total RC capacitance and may
influence the desired switching frequency.
2. The circuit board etch between the timing
resistor, capacitor, RC pin and ground must be
kept as short as possible to minimize noise
pickup and insure a stable oscillator waveform.
3. The ground lead of the capacitor must be routed
close to the ground lead of the MIC3838/9.
Current Sensing and Overcurrent Protection
The features are:
• Peak current limit
• Overcurrent limit
• Internal current sense discharge
• Front edge blanking
In current mode control, a PWM comparator uses the inductor
current signal and the error amplifier signal to determine the
operating duty cycle. In the MIC3838/9 the signal at the CS
pin is level shifted up before it reaches the PWM comparator
as shown in Figure 1. This allows operation of the error
amplifier and PWM comparator in a linear region.
There are two current limit thresholds in the MIC3838/9; peak
current limit and overcurrent limit. The normal operating
voltage at the ILIM pin is designed less than these thresholds.
A pulse-by-pulse current limit occurs when the inductor
current signal at the ILIM pin exceeds the peak current limit
threshold. The on-time is terminated for the remainder of the
switching cycle, regardless of whether OUTA or OUTB is
active.
If the signal at the ILIM pin goes past the peak threshold and
exceeds the overcurrent limit threshold, the overcurrent limit
comparator forces the soft start node to discharge and
initiates a soft start reset.
An internal FET discharges the RAMP and ILIM pins at the end
of the oscillator charge time. The FET turns on when the
voltage on the RC pin reaches the upper threshold (VDD/2)
and remains on for the duration of the RC pin discharge time
and for typically 100ns after the start of the next on-time
period. The 100ns period at the beginning of the on-time
implements a front edge blanking feature that prevents false
triggering of the PWM comparator due to noise spikes on the
leading edge of the current turn-on signal. The front edge
blanking also sets the minimum on-time for OUTA and OUTB.
The timing diagram for the RAMP pin is shown in Figure 4.
Soft Start
The soft start feature helps reduce surge currents at the
power supply input source. An internal current source and
capacitor ramp up from 0V to near VDD at a typical rate of
1V/ms. The soft start feature limits the output voltage of the
error amplifier at the COMP pin. As the soft start voltage rises,
it allows the COMP pin voltage to rise, which in turn allows the
duty cycle of the output drivers to increase. The internal soft
start voltage is discharged and remains discharged during
the following conditions:
1. The VDD voltage drops below the turn-off
threshold.
2. The voltage on the CS pin exceeds the overcurrent comparator threshold.
Once the internal soft start discharge FET is turned on, it
cannot be turned off until the internal soft start voltage drops
down below 0.5V. This insures a clean restart.
Oscillator
The oscillator operates at twice the switching frequency of
either OUTA or OUTB. The oscillator generates a sawtooth
waveform on the RC pin. The rising edge of the waveform is
controlled by the external resistor/capacitor combination.
The fall time is set by the on-resistance of the discharge FET
(see Figure 3). The fall time sets the delay (dead time)
between the turn-off of one output driver and the turn-on of the
other driver. A toggle flip-flop insures that drive signals to
OUTA and OUTB are alternated and therefore insures a
maximum duty cycle of less than 50% for each output driver.
Graphs of component values vs. oscillator frequency and
dead time are shown in the typical characteristic section of
this specification.
VDD
4
RC
VDD
2
S
Q
R
OSCILLATOR
OUTPUT
0.2V
Figure 3. Oscillator
The voltage source to the resistor/capacitor timing components is VDD. The internal turn-off comparator threshold in the
oscillator circuit is VDD/2. This allows the oscillator to track
changes in VDD and minimize frequency variations in the
oscillator. The oscillator frequency can be roughly approximated using the following formula:
1.41
R×C
Where: frequency is in Hz
Resistance is in ohms
Capacitance is in Farads.
FOSCILLATOR =
April 2005
9
MIC3838/3839
MIC3838/3839
Micrel
Max ON
time
Output Drivers
OUTA and OUTB are alternating output stages, which switch
at half the oscillator frequency. A toggle flip-flop in the
MIC3838/9 guarantee both outputs will not be on at the same
time. The RC discharge time is the dead time, where both
outputs are off. This provides an adjustable non-overlap time
to prevent shoot through currents and transformer saturation
in the power supply.
The output drivers are inhibited when VDD is below the
undervoltage threshold. Internal circuitry prevents the output
drivers from glitching high when VDD is first applied to the
MIC3838/9 controller.
Decoupling and PCB Layout
PCB layout is critical to achieve reliable, stable and efficient
operation. A ground plane is required to control EMI and
minimize the inductance in power, signal and return paths.
The following guidelines should be followed to insure proper
operation of the circuit:
• Low level signal and power grounds should be kept
separate and connected at only one location, preferably
the ground pin of the control IC. The ground signals for
the current sense, voltage feedback and oscillator
should be grouped together. The return signals for the
gate drives should be grouped together and a common
connection made at the ground pin of the controller. The
low level signals and their returns must be kept separate
from the high current and high voltage power section of
the power supply.
• Avoid running sensitive traces, such as the current
sense and voltage feedback signals next to or under
power components, such as the switching FETs and
transformer.
• If a current sense resistor is used, it’s ground end must
be located very close to the ground pin of the MIC3838/9
controller. Careful PCB layout is necessary to keep the
high current levels in the current sense resistor from
running over the low level signals in the controller.
• A minimum 1µF bypass capacitor must be connected
directly between the VDD and GND pins of the
MIC3838/9. An additional 0.1µF capacitor between the
VDD end of the oscillator frequency setting resistor and
the ground end of the oscillator capacitor may be
necessary if the resistor is a distance away from the
main 1µF bypass capacitor
dead time
RC Pin
Oscillator
Reset
dead time
Front edge blanking
RAMP Pin
Minimum ON time
OUTA
OUTB
Figure 4. Timing Diagram
Error Amplifier
The error amplifier is part of the voltage control loop of the
power supply. The FB pin is the inverting input to the error
amplifier. The non-inverting input is internally connected to a
reference voltage. The output of the error amplifier, COMP,
is connected to the PWM comparator. A voltage divider
between the error amplifier output (COMP pin) and the PWM
comparator allows the error amplifier to operate in a linear
region for better transient response. The output of the error
amplifier (COMP pin) is clamped at typically 3.65V to prevent
the COMP pin from rising up too high during startup or during
a transient condition. This feature improves the transient
response of the power supply.
MIC3838/3839
10
April 2005
MIC3838/3839
Micrel
Package Information
3.15 (0.122)
2.85 (0.114)
DIMENSIONS:
MM (INCH)
4.90 BSC (0.193)
3.10 (0.122)
2.90 (0.114)
1.10 (0.043)
0.94 (0.037)
0.30 (0.012)
0.15 (0.006)
0.50 BSC (0.020)
0.15 (0.006)
0.05 (0.002)
0.26 (0.010)
0.10 (0.004)
6° MAX
0° MIN
0.70 (0.028)
0.40 (0.016)
10-Pin MSOP (MM)
MICREL, INC.
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FAX
+ 1 (408) 474-1000
WEB
http://www.micrel.com
The information furnished by Micrel in this datasheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use.
Micrel reserves the right to change circuitry and specifications at any time without notification to the customer.
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 at Purchaser’s own risk and Purchaser agrees to fully indemnify
Micrel for any damages resulting from such use or sale.
© 2005 Micrel, Incorporated.
April 2005
11
MIC3838/3839