19-2115; Rev 0; 7/01
Current-Mode PWM Controllers with Integrated
Startup Circuit
The MAX5019/MAX5020 integrate all the building
blocks necessary for implementing DC-DC fixed-frequency power supplies. Either primary- or secondaryside regulation may be used to implement isolated or
nonisolated power supplies. These devices are currentmode controllers with an integrated high-voltage startup circuit suitable for telecom/industrial voltage range
power supplies. Current-mode control with leadingedge blanking simplifies control-loop design and internal ramp compensation circuitry stabilizes the current
loop when operating at duty cycles above 50%
(MAX5019). The MAX5019 allows 85% operating duty
cycle and can be used to implement flyback converters
whereas the MAX5020 limits the operating duty cycle to
less than 50% and can be used in single-ended forward converters. A high-voltage startup circuit allows
these devices to draw power directly from the 18V to
110V input supply during startup. The switching frequency is internally trimmed to 275kHz ±10%, thus
reducing magnetics and filter component costs.
The MAX5019/MAX5020 are available in 8-pin SO
packages.
Warning: The MAX5019/MAX5020 operate with high
voltages. Exercise caution.
Applications
Features
♦ Wide Input Range: (18V to 110V) or (13V to 36V)
♦ Isolated (without optocoupler) or Nonisolated
Power Supply
♦ Current-Mode Control
♦ Leading-Edge Blanking
♦ Internally Trimmed 275kHz ±10% Oscillator
♦ Low External Component Count
♦ Soft-Start
♦ High-Voltage Startup Circuit
♦ Pulse-by-Pulse Current Limiting
♦ Thermal Shutdown
♦ SO-8 Package
Ordering Information
PART
TEMP. RANGE
PIN-PACKAGE
MAX5019CSA*
0°C to +70°C
8-SO
MAX5019ESA*
-40°C to +85°C
8-SO
MAX5020CSA*
0°C to +70°C
8-SO
MAX5020ESA*
-40°C to +85°C
8-SO
*See Selector Guide at end of data sheet.
Telecom Power Supplies
Industrial Power Supplies
Networking Power Supplies
Isolated Power Supplies
Typical Operating Circuit
Pin Configuration
VIN
TOP VIEW
VOUT
V+
VDD
MAX5020
V+ 1
VDD 2
FB
3
MAX5019/
MAX5020
8
VCC
7
NDRV
6
GND
5
CS
VCC
CS
SS_SHDN
SS_SHDN 4
NDRV
GND
FB
8-SO
________________________________________________________________ Maxim Integrated Products
For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at
1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.
1
MAX5019/MAX5020
General Description
MAX5019/MAX5020
Current-Mode PWM Controllers with Integrated
Startup Circuit
ABSOLUTE MAXIMUM RATINGS
V+ to GND ……………………………………...……-0.3V to +120V
VDD to GND.………………………………….……….-0.3V to +40V
VCC to GND.………………….……………………-0.3V to +12.5V
FB, NDRV, SS_SHDN, CS to GND .……-0.3V to VCC + 0.3V
VDD and VCC Current …………………...…………………..20mA
NDRV Current Continuous...………………………………….25mA
NDRV Current for Less than 1µs..………….…………….……±1A
Continuous Power Dissipation (TA = +70°C)
8-Pin SO (derate 5.88mW/°C above +70°C) .………....471mW
Operating Temperature Range…………..……...-40°C to +85°C
Storage Temperature Range……………..…….-65°C to +150°C
Lead Temperature (soldering, 10s) ……………… ………+300°C
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
ELECTRICAL CHARACTERISTICS
(VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV
= open circuit, VFB = 3V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SUPPLY CURRENT
IV+(NS)
VDD = 0, V+ = 110V, driver not switching
V+ = 110V, VDD = 0, FB = GND,
driver switching
V+ = 110V, VDD = 13V, FB = GND
0.8
1.6
1.6
3.0
VDD = 36V, driver not switching
0.9
1.6
VDD = 36V, driver switching, FB = GND
2.1
3.0
V+ Shutdown Current
VSS_SHDN = 0, V+ = 110V
180
290
µA
VDD Shutdown Current
VSS_SHDN = 0
4
20
µA
V+ Supply Current
IV+(S)
V+ Supply Current After Startup
VDD Supply Current
IVDD(NS)
IVDD(S)
mA
µA
14
mA
PREREGULATOR/STARTUP
V+ Input Voltage
18
110
V
VDD Supply Voltage
13
36
V
INTERNAL REGULATORS (VCC)
VCC Output Voltage
VCC Undervoltage Lockout
VCC_UVLO
Powered from V+, ICC = 7.5mA, VDD = 0
7.5
9.8
12.0
V
Powered from VDD, ICC = 7.5mA
9.0
10.0
11.0
V
VCC falling
6.6
V
Peak Source Current
VCC = 11V (externally forced)
570
mA
Peak Sink Current
VCC = 11V (externally forced)
1000
mA
OUTPUT DRIVER
NRDV High-Side Driver
Resistance
ROH
VCC = 11V, externally forced,
NDRV sourcing 50mA
4
12
Ω
NDRV Low-Side Driver
Resistance
ROL
VCC = 11V, externally forced,
NDRV sinking 50mA
1.6
4
Ω
ERROR AMPLIFIER
FB Input Resistance
RIN
FB Input Bias Current
IFB
Error Amplifier Gain (Inverting)
50
VFB = VSS_SHDN
AVCL
Closed-Loop 3dB Bandwidth
FB Input Voltage Range
2
kΩ
±1
µA
-20
V/V
200
kHz
2
_______________________________________________________________________________________
3
V
Current-Mode PWM Controllers with Integrated
Startup Circuit
(VDD = 13V, a 10µF capacitor connects VCC to GND, VCS = 0, V+ = 48V, 0.1µF capacitor connected from SS_SHDN to GND, NDRV
= open circuit, VFB = 3V, TA = -40°C to +85°C, unless otherwise noted. Typical values are at TA = +25°C.)
PARAMETER
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
SLOPE COMPENSATION
Slope Compensation
(MAX5019 only)
26
VSCOMP
mV/µs
THERMAL SHUTDOWN
Thermal Shutdown Temperature
150
°C
Thermal Hysteresis
25
°C
CURRENT LIMIT
CS Threshold Voltage
VILIM
FB = GND
419
465
510
mV
1
µA
CS Input Bias Current
0 ≤ VCS ≤ 2V, FB = GND
Current Limit Comparator
Propagation Delay
50mV overdrive on CS, FB = GND
180
ns
CS Blanking Time
FB = GND, only PWM comparator is blanked
70
ns
-1
OSCILLATOR
Clock Frequency Range
FB = GND
Max Duty Cycle
247
275
302
MAX5019, FB = GND
75
85
MAX5020, FB = GND
44
50
VSS_SHDN = 0
2.0
kHz
%
SOFT-START
SS Source Current
ISSO
SS Sink Current
4.5
µA
6.5
1.0
Steady State Reference Voltage
at SS_SHDN
VSS_SHDN
Shutdown Threshold
mA
No external load
2.331
2.420
2.500
VSS_SHDN falling
0.25
0.37
0.41
VSS_SHDN rising
0.53
0.59
0.65
V
V
Typical Operating Characteristics
(V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.)
VSS_SHDN vs. TEMPERATURE
(AT THE END OF SOFT-START)
NDRV FREQUENCY (kHz)
1.001
1.000
FB = GND
276
275
274
0.999
-20
0
20
40
TEMPERATURE (°C)
60
80
80.9
80.8
FB = GND
80.7
80.6
80.5
273
-40
MAX5019 toc03
277
81.0
MAXIMUM DUTY CYCLE (%)
VFB = 4V
1.002
MAX5019 toc02
278
MAX5019 toc01
1.003
VSS_SHDN (V) (NORMALIZED TO VREF = 2.4V)
MAX5019
MAXIMUM DUTY CYCLE
vs. TEMPERATURE
NDRV FREQUENCY
vs. TEMPERATURE
80.4
-40
-20
0
20
40
TEMPERATURE (°C)
60
80
-40
-20
0
20
40
60
80
TEMPERATURE (°C)
_______________________________________________________________________________________
3
MAX5019/MAX5020
ELECTRICAL CHARACTERISTICS (continued)
Typical Operating Characteristics (continued)
(V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.)
47.2
47.0
1.59
1.58
1.57
1.56
0
20
60
V+ INPUT CURRENT vs.
TEMPERATURE (AFTER STARTUP)
V+ SHUTDOWN CURRENT
vs. TEMPERATURE
40
60
80
MAX5019 toc07
13.80
13.75
V+ = 110V, VDD = 13V, FB = GND
13.70
13.65
13.60
0
20
40
60
182.0
V+ = 110V, FB = SS_SHDN = GND
181.5
181.0
180.5
180.0
-20
0
20
40
60
TEMPERATURE (°C)
TEMPERATURE (°C)
NDRV RESISTANCE
vs. TEMPERATURE
CURRENT-LIMIT DELAY
vs. TEMPERATURE
HIGH-SIDE DRIVER
3.5
3.0
2.5
2.0
LOW-SIDE DRIVER
80
0.486
0.485
0.484
206
0
20
40
60
80
2.410
2.408
FB = GND, 100mV OVERDRIVE ON CS
204
-20
VSS_SHDN vs. VDD
202
200
198
196
2.406
2.404
194
2.402
192
1.5
60
FB = GND
-40
VSS_SHDN (V)
4.0
40
TEMPERATURE (°C)
208
CURRENT-LIMIT DELAY (ns)
4.5
20
0.487
80
210
MAX5019 toc10
5.0
0
0.483
-40
80
-20
0.488
MAX5019 toc11
-20
4.42
CS THRESHOLD VOLTAGE
vs. TEMPERATURE
179.0
-40
4.43
-40
179.5
13.50
4.44
TEMPERATURE (°C)
182.5
13.55
4.45
80
MAX5019 toc08
20
CS THRESHOLD VOLTAGE (V)
-20
TEMPERATURE (°C)
0
V+ = 110V
4.46
4.40
-40
40
TEMPERATURE (°C)
-20
V+ SHUTDOWN CURRENT (µA)
-40
4.47
4.41
1.55
46.8
V+ INPUT CURRENT (µA)
1.60
VDD = FB = SS_SHDN = GND
4.48
MAX5019 toc09
47.4
FB = VDD = GND
1.61
4.49
MAX5019 toc12
47.6
1.62
4.50
SOFT-START SOURCE CURRENT (µA)
1.63
V+ SUPPLY CURRENT (mA)
FB = GND
MAX5019 toc05
47.8
MAXIMUM DUTY CYCLE (%)
1.64
MAX5019 toc04
48.0
190
1.0
2.400
188
-40
4
SOFT-START SOURCE CURRENT
vs. TEMPERATURE
V+ SUPPLY CURRENT
vs. TEMPERATURE
MAX5019 toc06
MAX5020
MAXIMUM DUTY CYCLE
vs. TEMPERATURE
NDRV RESISTANCE (Ω)
MAX5019/MAX5020
Current-Mode PWM Controllers with Integrated
Startup Circuit
-20
0
20
40
TEMPERATURE (°C)
60
80
-40
-20
0
20
40
60
80
0
5
10
15
TEMPERATURE (°C)
_______________________________________________________________________________________
20
VDD (V)
25
30
35
40
Current-Mode PWM Controllers with Integrated
Startup Circuit
MAX5020
MAXIMUM DUTY CYCLE vs. VDD
269.5
269.0
268.5
FB = GND
268.0
47.8
VFB = 4V, CS = GND
47.7
47.6
47.5
DEVICE POWERED
FROM VDD
47.4
47.3
20
25
30
35
9.5
0
40
5
10
15
20
25
30
35
0
5
10
15
VDD (V)
VDD (V)
25
30
35
40
V+ SUPPLY CURRENT vs. V+ VOLTAGE
(AFTER STARTUP)
1.59
1.58
VFB = VDD = GND
1.56
1.55
1.54
1.53
16
14
V+ LEAKAGE CURRENT (µA)
MAX5019 toc16
1.60
1.57
20
VDD (V)
V+ SUPPLY CURRENT vs.
V+ VOLTAGE
V+ SUPPLY CURRENT (mA)
40
MAX5019 toc17
15
DEVICE POWERED
FROM V+
12
VDD = 13V, FB = GND
10
8
6
4
2
1.52
0
1.51
0
20
40
60
80
0 10 20 30 40 50 60 70 80 90 100 110
100
V+ VOLTAGE (V)
V+ VOLTAGE (V)
VCC VOLTAGE vs. VCC CURRENT
VCC VOLTAGE vs. VCC CURRENT
V+ = 110V, VFB = 4V
10.2
VDD = 36V
10.0
MAX5019 toc18
10.4
9.8
VDD = 13V
9.6
VDD = GND, VFB = 4V
9.9
V+ = 110V
V+ = 90V
V+ = 72V
V+ = 48V
9.8
VCC VOLTAGE (V)
10.0
MAX5019 toc19
10
VCC VOLTAGE (V)
5
FB = GND
9.8
9.6
47.0
0
9.9
9.7
DEVICE POWERED
FROM V+
47.1
267.0
DEVICE POWERED FROM VDD
10.0
47.2
267.5
10.1
VCC (V)
270.0
VCC vs. VDD
10.2
MAX5019 toc14
47.9
MAXIMUM DUTY CYCLE (%)
270.5
NDRV FREQUENCY (kHz)
48.0
MAX5019 toc13
271.0
MAX5019 toc15
NDRV FREQUENCY vs. VDD
9.7
9.6
9.5
V+ = 36V
9.4
V+ = 24V
9.3
9.4
9.2
9.2
9.1
9.0
9.0
0
5.0
10.0
15.0
VCC CURRENT (mA)
20.0
0
5.0
10.0
15.0
20.0
VCC CURRENT (mA)
_______________________________________________________________________________________
5
MAX5019/MAX5020
Typical Operating Characteristics (continued)
(V+ = 48V, VDD = 13V, CS = GND, NRDV is open circuit, TA = +25°C, unless otherwise noted.)
Current-Mode PWM Controllers with Integrated
Startup Circuit
MAX5019/MAX5020
Pin Description
PIN
NAME
FUNCTION
V+
High-Voltage Startup Input. Connect directly to an input voltage between 18V to 110V. Connects
internally to a high-voltage linear regulator that generates VCC during startup.
2
VDD
VDD is the Input of the Linear Regulator that Generates VCC. For supply voltages less than 36V, VDD
and V+ can both be connected to the supply. For supply voltages greater than 36V, VDD receives
its power from the tertiary winding of the transformer and accepts voltages from 13V to 36V. Bypass
to GND with a 4.7µF capacitor.
3
FB
Input of the Fixed-Gain Inverting Amplifier. Connect a voltage-divider from the regulated output to
this pin. The noninverting input of the amplifier is referenced to 2.4V.
4
SS_SHDN
Soft-Start Timing Capacitor Connection. Ramp time to full current limit is approximately 0.45ms/nF.
This pin is also the reference voltage output. Bypass with a minimum 10nF capacitor to GND. The
device goes into shutdown when SS_SHDN is pulled below 0.25V.
5
CS
6
GND
Ground
7
NDRV
Gate Drive. Drives a high-voltage external N-channel power MOSFET.
8
VCC
1
Current Sense Input. Turns power switch off if VCS rises above 465mV for cycle-by-cycle current
limiting. CS is also the feedback for the current-mode controller. CS is connected to the PWM
comparator through a leading-edge blanking circuit.
Regulated IC Supply. Provides power for the entire IC. VCC is regulated from VDD during normal
operation and from V+ during startup. Bypass VCC with a 10µF tantalum capacitor in parallel with
0.1µF ceramic capacitor to GND.
Detailed Description
Use the MAX5019/MAX5020 PWM current-mode controllers to design flyback- or forward-mode power supplies. Current-mode operation simplifies control-loop
design while enhancing loop stability. An internal highvoltage startup regulator allows the device to connect
directly to the input supply without an external startup
resistor. Current from the internal regulator starts the
controller. Once the tertiary winding voltage is established the internal regulator is switched off and bias
current for running the IC is derived from the tertiary
winding. The internal oscillator is set to 275kHz and
trimmed to ±10%. This permits the use of small magnetic components to minimize board space. Both the
MAX5019 and MAX5020 can be used in power supplies providing multiple output voltages. A functional
diagram of the IC is shown in Figure 1. Typical applications circuits for forward and flyback topologies are
shown in Figure 2 and Figure 3, respectively. For isolated flyback power supplies use the circuit of Figure 4.
Current-Mode Control
The MAX5019/MAX5020 offer current-mode control
operation with added features such as leading-edge
blanking with dual internal path that only blanks the
6
sensed current signal applied to the input of the PWM
comparator. The current limit comparator monitors the
CS pin at all times and provides cycle-by-cycle current
limit without being blanked. The leading-edge blanking
of the CS signal prevents the PWM comparator from
prematurely terminating the on cycle. The CS signal
contains a leading-edge spike that is the result of the
MOSFET gate charge current, capacitive and diode
reverse recovery current of the power circuit. Since this
leading-edge spike is normally lower than the current
limit comparator threshold, current limiting is not
blanked and cycle-by-cycle current limiting is provided
under all conditions.
Use the MAX5019 in discontinuous flyback applications
where wide line voltage and load current variation is
expected. Use the MAX5020 for single transistor forward converters where the maximum duty cycle must
be limited to less than 50%.
Under certain conditions it may be advantageous to
use a forward converter with greater than 50% duty
cycle. For those cases use the MAX5019. The large
duty cycle results in much lower operating primary
RMS currents through the MOSFET switch and in most
cases a smaller output filter inductor. The major disad-
_______________________________________________________________________________________
Current-Mode PWM Controllers with Integrated
Startup Circuit
MAX5019/MAX5020
VDD
VDD-OK
V+
IN
IN
HIGHVOLTAGE
REGULATOR
GND
EN
BIAS
WINDING
REGULATOR
OUT
EN
OUT
0.7V
VCC
MAX5019 ONLY
UVLO
SLOPE
COMPENSATION
26mV/µs
6.6V
275kHz
OSCILLATOR
VCC
R
NDRV
Q
26mV/µs
80%/50%
DUTY CYCLE
CLAMP
1MΩ
FB
S
∑
50kΩ
ILIM
PWM
125mV
CS
ERROR
AMP
5kΩ
VCC
SS_SHDN
70ns
BLANKING
4µA
3R
2.4V
BUF
R
0.25V
Figure 1. Functional Diagram
_______________________________________________________________________________________
7
MAX5019/MAX5020
Current-Mode PWM Controllers with Integrated
Startup Circuit
1N4148
6
VIN
(36V TO 72V)
NT
N
14 R
CMHD2003
V+
VDD
CDD
4.7µF
VCC
CCC
10µF
MAX5020
NDRV
CIN
3✕
0.47µF NP
14
SBL204OCT
NS
5
L1
4.7µH
VOUT
5V/10A
COUT
3✕
560µF
20Ω
0.1µF
1nF
M1
IRF640N
R1
2kΩ
CS
100Ω
SS_SHDN
CSS
0.1µF
RSENSE
100mΩ
GND
FB
R2
2kΩ
CFB
(OPTIONAL)
Figure 2. Forward Converter
vantage to this is that the MOSFET voltage rating must
be higher and that slope compensation must be provided to stabilize the inner current loop. The MAX5019
provides internal slope compensation.
Internal Regulators
The internal regulators of the MAX5019/MAX5020
enable initial startup without a lossy startup resistor and
regulate the voltage at the output of a tertiary (bias)
winding to provide power for the IC. At startup V+ is
regulated down to VCC to provide bias for the device.
The VDD regulator then regulates from the output of the
tertiary winding to VCC. This architecture allows the tertiary winding to only have a small filter capacitor at its
output thus eliminating the additional cost of a filter
inductor.
When designing the tertiary winding calculate the number of turns so the minimum reflected voltage is always
higher than 12.7V. The maximum reflected voltage
must be less than 36V.
To reduce power dissipation the high-voltage regulator
is disabled when the VDD voltage reaches 12.7V. This
greatly reduces power dissipation and improves efficiency. If V CC falls below the undervoltage lockout
threshold (VCC = 6.6V), the low-voltage regulator is dis-
8
abled, and soft-start is reinitiated. In undervoltage lockout the MOSFET driver output (NDRV) is held low.
If the input voltage range is between 13V and 36V, V+
and VDD may be connected to the line voltage provided that the maximum power dissipation is not exceeded. This eliminates the need for a tertiary winding.
Undervoltage Lockout (UVLO), Soft-Start,
and Shutdown
The soft-start feature of the MAX5019/MAX5020 allows
the load voltage to ramp up in a controlled manner,
thus eliminating output voltage overshoot.
While the part is in UVLO, the capacitor connected to
the SS_SHDN pin is discharged. Upon coming out of
UVLO an internal current source starts charging the
capacitor to initiate the soft-start cycle. Use the following equation to calculate total soft-start time:
tstartup = 0.45
ms
× Css
nF
where CSS is the soft-start capacitor as shown in Figure 2.
Operation begins when VSS_SHDN ramps above 0.6V.
When soft-start has completed, VSS_SHDN is regulated
_______________________________________________________________________________________
Current-Mode PWM Controllers with Integrated
Startup Circuit
V+
VDD
VOUT
CCC
COUT
CIN
NP
CDD
VCC
MAX5019
MAX5020
MAX5019/MAX5020
NT
VIN
NS
M1
NDRV
CS
100Ω
SS_SHDN
RSENSE
R1
CSS
GND
FB
R2
Figure 3. Nonisolated Flyback Converter
NT
VOUT
VIN
V+
VDD
COUT
CIN
NP
CDD
NS
R1
FB
R2
MAX5019
MAX5020
VCC
NDRV
M1
CS
CCC
100Ω
RSENSE
SS_SHDN
GND
CSS
Figure 4. Isolated Flyback Converter
to 2.4V, the internal voltage reference. Pull VSS_SHDN
below 0.25V to disable the controller.
Undervoltage lockout shuts down the controller when
VCC is less than 6.6V. The regulators for V+ and the reference remain on during shutdown.
Current-Sense Comparator
The current-sense (CS) comparator and its associated
logic limit the peak current through the MOSFET.
Current is sensed at CS as a voltage across a sense
resistor between the source of the MOSFET and GND.
To reduce switching noise, connect CS to the external
MOSFET source through a 100Ω resistor or an RC low-
_______________________________________________________________________________________
9
MAX5019/MAX5020
Current-Mode PWM Controllers with Integrated
Startup Circuit
pass filter (Figures 2, 3). Select the current-sense resistor, RSENSE according to the following equation:
RSENSE = 0.465V / ILimPrimary
where ILimPrimary is the maximum peak primary-side
current.
When VCS > 465mV, the power MOSFET switches off.
The propagation delay from the time the switch current
reaches the trip level to the driver turn-off time is 180ns.
Internal Error Amplifier
The MAX5019/MAX5020 include an internal error amplifier that can be used to regulate the output voltage in
the case of a nonisolated power supply (see Figure 2).
Calculate the output voltage using the following equation:
R
VOUT = 1+ 1 × VREF
R2
where VREF = 2.4V.
Choose R1//R2 VEA - VREF - VSCOMP
where IPRIMARY is the current through the N-channel
MOSFET, VREF is the 2.4V internal reference, VEA is the
output voltage of the internal amplifier, and VSCOMP is
a ramp function starting at 0 and slewing at 26mV/µs
(MAX5019 only). When using the MAX5019 in a forward-converter configuration the following condition
must be met to avoid control-loop subharmonic oscillations:
NS k × RSENSE × VOUT
×
= 26mV / µs
L
NP
where k = 0.75 to 1, and NS and NP are the number of
turns on the secondary and primary side of the transformer, respectively. L is the output filter inductor. This
makes the output inductor current downslope as referenced across RSENSE equal to the slope compensation. The controller responds to transients within one
cycle when this condition is met.
N-Channel MOSFET Gate Driver
NDRV drives an N-channel MOSFET. NDRV sources
and sinks large transient currents to charge and discharge the MOSFET gate. To support such switching
transients, bypass VCC with a ceramic capacitor. The
average current as a result of switching the MOSFET is
the product of the total gate charge and the operating
frequency. It is this current plus the DC quiescent current that determines the total operating current.
Applications Information
Design Example
The following is a general procedure for designing a
forward converter using the MAX5020.
1) Determine the requirements.
2) Set the output voltage.
3) Calculate the transformer primary to secondary
winding turns ratio.
4) Calculate the reset to primary winding turns ratio.
5) Calculate the tertiary to primary winding turns
ratio.
6) Calculate the current-sense resistor value.
7) Calculate the output inductor value.
8) Select the output capacitor.
The circuit in Figure 2 was designed as follows:
______________________________________________________________________________________
Current-Mode PWM Controllers with Integrated
Startup Circuit
R
VOUT ≅ VREF 1+ 1
R2
R1 // R2
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