LM5021
AC-DC Current Mode PWM Controller
LM5021 AC-DC Current Mode PWM Controller
June 2005
General Description
The LM5021 off-line pulse width modulation (PWM) controller contains all of the features needed to implement highly
efficient off-linesingle-ended flybackand forward powerconverters using current-mode control. The LM5021 features
include an ultra-low (25 µA) start-up current, which minimizes power losses in the high voltage start-up network. A
skip cycle mode reduces powerconsumption with light loads
for energy conserving applications (ENERGY STAR
CECP, etc.). Additional features include under-voltage lockout, cycle-by-cycle current limit, hiccup mode overload protection, slope compensation, soft-start and oscillator synchronization capability. This high performance 8-pin IC has
total propagation delays less than 100nS and a 1MHz capable oscillator that is programmed with a single resistor.
Simplified Application Diagram
Features
n Ultra Low Start-up Current (25 µA maximum)
n Current Mode Control
n Skip Cycle Mode for Low Standby Power
n Single Resistor Programmable Oscillator
n Synchronizable Oscillator
n Adjustable Soft-start
n Integrated 0.7A Peak Gate Driver
®
,
n Direct Opto-Coupler Interface
n Maximum Duty Cycle Limiting (80% for LM5021-1 or
50% for LM5021-2)
n Slope Compensation for (LM5021-1 Only)
n Under Voltage Lockout (UVLO) with Hysteresis
n Cycle-by-Cycle Over-Current Protection
n Hiccup Mode for Continuous Overload Protection
n Leading Edge Blanking of Current Sense Signal
n Packages: MSOP-8 or MDIP-8
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© 2005 National Semiconductor Corporation DS201442 www.national.com
Connection Diagram
LM5021
Top View
MSOP-8 and MDIP-8
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Ordering Information
Order Number Description Package Type Supplied As
LM5021MM-1 80% Duty Cycle Limit MSOP-8 1000 Units on Tape and Reel
LM5021MMX-1 80% Duty Cycle Limit MSOP-8 3500 Units on Tape and Reel
LM5021NA-1 80% Duty Cycle Limit MDIP-8 40 Units per Rail
LM5021MM-2 50% Duty Cycle Limit MSOP-8 1000 Units on Tape and Reel
LM5021MMX-2 50% Duty Cycle Limit MSOP-8 3500 Units on Tape and Reel
LM5021NA-2 50% Duty Cycle Limit MDIP-8 40 Units per Rail
Pin Description
PIN NAME DESCRIPTION APPLICATION INFORMATION
1 COMP Control input for the Pulse Width Modulator
and Hiccup comparators.
2 VIN Input voltage. Input to start-up regulator. The VIN pin is clamped at
3 VCC Output of a linear bias supply regulator.
Nominally 8.5V.
4 OUT MOSFET gate driver output. High current output to the external MOSFET gate input
5 GND Ground return.
6 CS Current Sense input. Current sense input for current mode control and
7 RT / SYNC Oscillator timing resistor pin and
synchronization input.
8 SS Soft-start / Hiccup time An external capacitor and an internal 22 µA current
COMP pull-up is provided by an internal 5K resistor
which may be used to bias an opto-coupler transistor.
36V by an internal zener diode.
VCC provides bias to controller and gate drive sections
of the LM5021. An external capacitor must be connected
from this pin to ground.
with source/sink current capability of 0.3A and 0.7A
respectively.
over-current protection. Current limiting is accomplished
using a dedicated current sense comparator. If the CS
comparator input exceeds 0.5 Volts the OUT pin
switches low for cycle-by-cycle current limit. CS is held
low for 90ns after OUT switches high to blank the
leading edge current spike.
An external resistor connected from RT to GND sets the
oscillator frequency. This pin will also accept
synchronization pulses from an external clock.
source set the soft-start ramp. The soft -start capacitor
controls both the soft-start rate and the hiccup mode
period.
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LM5021
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
ESD Rating (Note 2)
Human Body Model 2kV
Storage Temperature -65˚C to +150˚C
Operating Junction Temperature +150˚C
VIN to GND -0.3V to 30V
VIN Clamp Continuous Current 5mA
Operating Ratings (Note 1)
CS to GND -0.3V to 1.25V
RT to GND -0.3V to 5.5V
All other pins to GND -0.3V to 7.0V
Electrical Characteristics Specifications in standard type face are for T
type apply over the full Operating Junction Temperature Range. Unless otherwise specified: V
VIN Voltage (Note 5) 8V to 30V
Junction Temperature -40˚C to +125˚C
= +25˚C and those in boldface
J
= 15V, RT= 44.2KΩ.
IN
(Note 3)
Symbol Parameter Conditions Min Typ Max Unit
STARTUP CIRCUIT
Start Up Current Before VCC Enable 18 25 µA
VCC Regulator enable threshold 17 20 23 V
VCC Regulator disable
7.25 V
threshold
VIN ESD Clamp voltage I = 5mA 30 36 40 V
I
VIN
Operating supply current COMP = 0VDC 2.5 3.75 mA
VCC SUPPLY
Controller enable threshold 6.5 7 7.5 V
Controller disable threshold 5.3 5.8 6.3 V
VCC regulated output No External Load 8 8.5 9 V
VCC dropout voltage (VIN -
I=5mA 1.7 V
VCC)
VCC regulator current limit VCC = 7.5V (Note
15 22 mA
4)
SKIP CYCLE MODE COMPARATOR
Skip Cycle mode enable
1
⁄3[COMP - 1.25V] 75 125 175 mV
threshold
Skip Cycle mode hysteresis 5 mV
CURRENT LIMIT
CS limit to OUT delay CS stepped from 0
35 ns
to 0.6V, time to OUT
transition low, C
load
=0.
CS limit threshold 0.45 0.5 0.55 V
Leading Edge Blanking time 90 ns
CS blanking sinking impedance 35 55 Ω
SOFT-START
V
SS-OCV
SS pin open-circuit voltage 4.3 5.2 6.1 V
Soft-start Current Source 15 22 30 µA
Soft-start to COMP Offset 0.35 0.55 0.75 V
COMP sinking impedance During SS ramp 60 Ω
OSCILLATOR
Frequency1 (RT = 44.2K) 135 150 165 kHz
Frequency2 (RT = 13.3K) 440 500 560 kHz
Sync threshold 2.4 3.2 3.8 V
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Electrical Characteristics Specifications in standard type face are for T
apply over the full Operating Junction Temperature Range. Unless otherwise specified: V
LM5021
3) (Continued)
= +25˚C and those in boldface type
J
= 15V, RT= 44.2KΩ. (Note
IN
Symbol Parameter Conditions Min Typ Max Unit
PWM COMPARATOR
COMP to OUT delay COMP set to 2V
20 ns
CS stepped 0 to
0.4V, time to OUT
transition low, C
load
=0.
Min Duty Cycle COMP = 0V 0 %
Max Duty Cycle (-1 Device) 75 80 85 %
Max Duty Cycle (-2 Device) 50 %
COMP to PWM comparator gain 0.33
COMP Open Circuit Voltage 4.2 5.1 6 V
COMP at Max Duty Cycle 2.75 V
COMP Short Circuit Current COMP = 0V 0.6 1.1 1.5 mA
SLOPE COMPENSATION
Slope Comp Amplitude
(LM5021-1 only)
CS pin to PWM
Comparator offset at
70 90 110 mV
maximum duty cycle
OUTPUT SECTION
OUT High Saturation IOUT = 50mA, VCC
0.6 1.1 V
- OUT
OUT Low Saturation IOUT = 100mA 0.3 1 V
Peak Source Current OUT = VCC/2. 0.3 A
Peak Sink Current OUT = VCC/2. 0.7 A
Rise time C
Fall time C
= 1nF 25 ns
load
= 1nF 10 ns
load
HICCUP MODE
V
V
I
I
OVLD
V
RST
DTCS
OVCS
Over load detection threshold COMP pin V
Hiccup mode threshold SS pin V
HIC
SS-OCV
SS-OCV
– 0.8 V
– 0.8 V
SS-OCV
SS-OCV
– 0.6 V
– 0.6 V
SS-OCV
SS-OCV
Hiccup mode Restart threshold SS pin 0.1 0.3 0.5 V
Dead-time current source 0.1 0.25 0.4 µA
Overload detection timer current
6 10 14 µA
– 0.4 V
– 0.4 V
source
THERMAL RESISTANCE
θ
θ
Note 1: Absolute Maximum Ratingsare limitsbeyond which damageto thedevice may occur. Operating Ratingsare conditions underwhich operationof the device
is intended to be functional. For guaranteed specifications and test conditions, see the Electrical Characteristics.
Note 2: The human body model is a 100 pF capacitor discharged through a 1.5kΩ resistor into each pin.
Note 3: Min and Max limits are 100% production tested at 25˚C. Limits over the operating temperature range are guaranteed through correlation using Statistical
Quality Control (SQC) methods. Limits are used to calculate National’s Average Outgoing Quality Level (AOQL).
Note 4: Device thermal limitations may limit usable range.
Note 5: After initial turn-on at VIN = 20V.
MSOP-8 Junction to Ambient 0 LFM 200 ˚C/W
JA
MDIP-8 Junction to Ambient 0 LFM 107 ˚C/W
JA
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Simplified Block Diagram
LM5021
FIGURE 1.
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