May 1997
ML4823
*
High Frequency Power Supply Controller
FEATURES
s
Practical operation at switching frequencies to 1.0MHz
s
High current (2A peak) totem pole output
s
Wide bandwidth error amplifier
s
Fully latched logic with double pulse suppression
s
Pulse-by-pulse current limiting
s
Soft start and max. duty cycle control
s
Under voltage lockout with hysteresis
s
5.1V trimmed bandgap reference
s
Low start-up current (1.1mA)
s
Pin compatible improved replacement for UC3823
s
Fast shut down path from current limit to output
s
Soft start latch ensures full soft start cycle
s
Outputs pull low for undervoltage lockout
GENERAL DESCRIPTION
The ML4823 High Frequency PWM Controller is an IC
controller optimized for use in Switch Mode Power
Supply designs running at frequencies to 1MHz.
Propagation delays are minimal through the comparators
and logic for reliable high frequency operation while slew
rate and bandwidth are maximized on the error amplifier.
This controller is designed for single-ended applications
using voltage or current mode and provides for input
voltage feed forward.
A 1V threshold current limit comparator provides cycle-
by-cycle current limit and exceeding a 1.4V threshold
initiates a soft-start cycle. The soft start pin doubles as a
maximum duty cycle clamp. All logic is fully latched to
provide jitter-free operation and prevent multiple pulsing.
An under-voltage lockout circuit with 800mV of hysteresis
assures low startup current and drives the outputs low
during fault conditions.
This controller is an improved second source for the
UC3823 controller; however, the ML4823 includes
features not found on the 3823. These features are set in
*Some Packages Are Obsolete
italics.
BLOCK DIAGRAM
(Pin Configuration Shown for 16-Pin Version)
5
6
OSC
R
T
C
T
4
7
3
RAMP
E/A OUT
COMP
+
+
1.25V
S
R
Q
13
14
12
ERROR
AMP
+
2
1
NI
INV
POWER V
C
OUTPUT
POWER GND
CLOCK OUT
8
SOFT START
V+
+
1V
11
9
I
LIM
REF
I
LIM
/S.D.
+
1.4V
+
S
R
Q
4V
5.1V
+
16
5.1V V
REF
+
ENABLE
V
REF
15
V
CC
UNDER
VOLTAGE
LOCKOUT
9V
V
REF
GEN
10
SIGNAL GND
INTERNAL
BIAS
1
ML4823
2
PIN CONFIGURATION
PIN
NAME
FUNCTION
PIN
NAME
FUNCTION
1 (2)
INV
Inverting input to error amp.
2 (3)
NI
Non-inverting input to error amp.
3 (4)
E/A OUT Output of error amplifier and input to
main comparator.
4 (5)
CLOCK
Oscillator output.
5 (7)
R
T
Timing resistor for oscillator -- sets
charging current for oscillator timing
capacitor (pin 6).
6 (8)
C
T
Timing capacitor for oscillator.
7 (9)
RAMP
Non-inverting input to main
comparator. Connected to C
T
for
Voltage mode operation or to current
sense resistor for current mode.
8 (10)
SS
Normally connected to soft start
capacitor.
PIN DESCRIPTION
(Pin Numbers in Parentheses are for PLCC Version)
9 (12) I
LIM
/S.D.
Current limit sense pin. Normally
connected to current sense resistor.
10 (13) GND
Analog signal ground.
11 (14) I
LIM
REF
Reference input for cycle-by-cycle
current limit comparator.
12 (15) PWR GND Return for the high current totem
pole output.
13 (17) V
C
Positive supply for the high current
totem pole output.
14 (18) OUT B
High current totem pole output.
15 (19) V
CC
Positive supply for the IC.
16 (20) 5.1V REF
Buffered output for the 5.1V voltage
reference.
INV
NI
E/A OUT
CLOCK
R
T
C
T
RAMP
SS
5.1V REF
V
CC
OUTPUT
V
C
PWR GND
I
LIM
REF
GND
I
LIM
/S.D.
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
TOP VIEW
3
2
1
20
19
18
17
16
15
14
9
10
11
12
13
4
5
6
7
8
RAMP
SS
NC
I
LIM
/S.D.
GND
NI
INV
NC
5.1V
REF
V
CC
OUTPUT
V
C
NC
PWR GND
I
LIM
REF
E/A OUT
CLOCK
NC
R
T
C
T
TOP VIEW
ML4823
16-PIN DIP (P16)
16-PIN SOIC (S16W)
ML4823
20-PIN PLCC (Q20)
ML4823
3
ABSOLUTE MAXIMUM RATINGS
Absolute maximum ratings are those values beyond which
the device could be permanently damaged. Absolute
maximum ratings are stress ratings only and functional
device operation is not implied.
Supply Voltage (V
C
, V
CC
) .......................................... 30V
OUTPUT Current, Source or Sink
DC ....................................................................... 0.5A
Pulse (0.5
s) ......................................................... 2.0A
Analog Inputs
(INV, NI, RAMP, SS, I
LIM
) .................. GND 0.3V to 6V
CLOCK OUTPUT Current ....................................... 5mA
F/A OUT Current ...................................................... 5mA
SOFT START Sink Current ...................................... 20mA
R
T
Charging Current ................................................ 5mA
OPERATING CONDITIONS
Temperature Range
ML4823C .................................................. 0
C to 70
C
ML4823I ................................................ 40
C to 85
C
Junction Temperature ............................................. 125
C
Storage Temperature Range ..................... 65
C to 150
C
Lead Temperature (Soldering 10 sec.) ..................... 260
C
Thermal Resistance (
JA
)
Plastic DIP ....................................................... 80
C/W
Plastic SOIC .................................................. 105
C/W
Plastic Chip Carrier (PLCC) .............................. 78
C/W
ELECTRICAL CHARACTERISTICS
Unless otherwise specified, R
T
= 3.65k
, C
T
= 1000pF, T
A
= Operating Temperature Range, V
CC
= 15V. (Note 1)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
OSCILLATOR
Initial Accuracy
T
J
= 25
C,
360
400
440
kHz
Voltage Stability
10V
V
CC
30V,
0.2
2
%
Temperature Stability
5
%
Total Variation
Line, temp.
340
460
kHz
Clock Out High
3.9
4.5
V
Clock Out Low
2.3
2.9
V
Ramp Peak
2.6
2.8
3.0
V
Ramp Valley
0.7
1.0
1.25
V
Ramp Valley to Peak
1.6
1.8
2.0
V
REFERENCE
Output Voltage
T
J
= 25
C, I
O
= 1mA
5.025
5.10
5.175
V
Line Regulation
10V
V
CC
30V
2
20
mV
Load Regulation
1mA
I
O
10mA
5
20
mV
Temperature Stability
40
C
T
J
150
C,
0.2
0.4
%
Total Variation
Line, load, temp.
4.975
5.225
V
Output Noise Voltage
10Hz to 10kHz
50
V
Long Term Stability
T
J
= 125
C, 1000 hrs,
5
25
mV
Short Circuit Current
V
REF
= 0V
15
50
100
mA
ERROR AMPLIFIER
Input Offset Voltage
30
mV
Input Bias Current
0.6
3
A
Input Offset Current
0.1
1
A
Open Loop Gain
1
V
O
4V
50
95
dB
ML4823
4
ELECTRICAL CHARACTERISTICS
(Continued)
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
ERROR AMPLIFIER (Continued)
CMRR
1.5
V
CC
5.5V
50
80
dB
PSRR
10
V
CC
30V
70
100
dB
Output Sink Current
V
E/A OUT
= 1V
1
2.5
mA
Output Source Current
V
E/A OUT
= 4V
0.5
1.3
mA
Output High Voltage
I
E/A OUT
= 0.5mA
4.0
4.7
5.0
V
Output Low Voltage
I
E/A OUT
= 1mA
0
0.5
1.0
V
Unity Gain Bandwidth
3
5.5
MHz
Slew Rate
6
12
V/
s
PWM COMPARATOR
RAMP Bias Current
V
RAMP
= 0V
1
5
A
Duty Cycle Range
0
80
%
E/A OUT Zero DC Threshold
V
RAMP
= 0V
1.1
1.25
V
Delay to Output
50
80
ns
SOFT START
Charge Current
V
SOFT START
= 0.5V
3
9
20
A
Discharge Current
V
SOFT START
= 1V
1
mA
CURRENT LIMIT/SHUTDOWN
I
LIM
Bias Current
0V
I
LIM
4V
10
A
Current Limit Offset
I
LIM
REF = 1.1V
0
15
mV
I
LIM
REF Common Mode Range
1.0
1.25
V
Shutdown Threshold
1.25
1.40
1.55
V
Delay to Output
50
80
ns
OUTPUT
Output Low Level
I
OUT
= 20mA
0.25
0.40
V
I
OUT
= 200mA
1.2
2.2
V
Output High Level
I
OUT
= 20mA
12.8
13.5
V
I
OUT
= 200mA
12.0
13.0
V
Collector Leakage
V
C
= 30V
100
500
A
Rise/Fall Time
C
L
= 1000pF
30
60
ns
UNDER VOLTAGE LOCKOUT
Start Threshold
8.8
9.2
9.7
V
UVLO Hysteresis
0.4
0.8
1.2
V
SUPPLY
Start Up Current
V
CC
= 8V
1.1
2.5
mA
I
CC
INV, RANP, I
LIM
= 0V
22
33
mA
NI = 1V
Note 1: Limits are guaranteed by 100% testing, sampling, or correlation with worst-case test conditions.
ML4823
5
FUNCTIONAL DESCRIPTION
OSCILLATOR
The ML4823 oscillator charges the external capacitor (C
T
)
with a current (I
SET
) equal to 3/R
SET
. When the capacitor
voltage reaches the upper threshold (Ramp Peak), the
comparator changes state and the capacitor discharges to
the lower threshold (Ramp Valley) through Q1. While the
capacitor is discharging, Q2 provides a high pulse.
The oscillator period can be described by the following
relationship:
T
OSC
= T
RAMP
+ T
DEADTIME
where: T
RAMP
= C (Ramp Valley to Peak)/I
SET
and:
T
DEADTIME
= C (Ramp Valley to Peak)/I
Q1
+
5.1V
Q1
R
T
R
T
I
SET
C
T
I
SET
400
A
I
Q1
3V
Q
2
CLOCK
OUT
CLOCK OUT
T
D
C
T
RAMP PEAK
RAMP VALLEY
Figure 2. Oscillator Timing Resistance vs Frequency
Figure 4. Oscillator Deadtime vs C
T
(3k
R
T
100k
)
30
25
20
15
10
5
0
0.47
1.0
2.2
4.7
10.0
22
C
T
(nF)
T
D
(
s)
47
100
160
140
120
100
80
10k
100k
1M
FREQ (Hz)
T
D
(ns)
1.0nF
470pF
Figure 3. Oscillator Deadtime vs Frequency
Figure 1. Oscillator Block Diagram
100k
10k
1k
0.1k
1k
10k
100k
1M
10M
FREQ (Hz)
RT (OHMS)
100nF
47nF
22nF
10nF
4.7nF
2.2nF
1nF
470pF
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