ETC CS51221/D

CS51221
Enhanced Voltage Mode
PWM Controller
The CS51221 fixed frequency feed forward voltage mode PWM
controller contains all of the features necessary for basic voltage mode
operation. This PWM controller has been optimized for high
frequency primary side control operation. In addition, this device
includes such features as: Soft Start, accurate duty cycle limit control,
less than 50µA startup current, over and under voltage protection, and
bidirectional synchronization. The CS51221 is available in a 16 lead
SO narrow surface mount package.
16
1
SO–16
D SUFFIX
CASE 751B
PIN CONNECTIONS AND
MARKING DIAGRAM
1
16
GATE
ISENSE
SYNC
FF
UV
OV
RTCT
ISET
A
WL, L
YY, Y
WW, W
CS51221
AWLYWW
Features
1.0 MHz Frequency Capability
Fixed Frequency Voltage Mode Operation, with Feed Forward
Thermal Shutdown
Undervoltage Lock–Out
Accurate Programmable Max Duty Cycle Limit
1.0 A Sink/Source Gate Drive
Programmable Pulse–By–Pulse Overcurrent Protection
Leading Edge Current Sense Blanking
75 ns Shutdown Propagation Delay
Programmable Soft Start
Undervoltage Protection
Overvoltage Protection with Programmable Hysteresis
Bidirectional Synchronization
25 ns GATE Rise and Fall Time (1.0 nF Load)
3.3 V 3% Reference Voltage Output
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
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VC
PGND
VCC
VREF
LGND
SS
COMP
VFB
= Assembly Location
= Wafer Lot
= Year
= Work Week
ORDERING INFORMATION
Device
 Semiconductor Components Industries, LLC, 2001
February, 2001 – Rev. 5
1
Package
Shipping
CS51221ED16
SO–16
48 Units/Rail
CS51221EDR16
SO–16
2500 Tape & Reel
Publication Order Number:
CS51221/D
100
BAS21
VIN (36 V to 72 V)
51 k
1.0 µF
FZT688
22 µF 18 V
11 V
T3
100:1
0.1 µF
10
24.3 k
510 k
T2
2:5
T1
4:1
10 k
MBRB2545CT
VOUT
(5.0 V/5.0 A)
0.22 µF
200
VREF
UV
COMP
OV
2200 pF
VFB
1.0 µF
RTCT
SYNC
CS51221
10 k
10
VCC
VC
4.3 k
ISET
D11
BAS21
SGND
680 pF
D13
V33MLA1206A23
100 pF
13 k
100 µF
CS51221
2
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Figure 1. Application Diagram, 36 V–72 V to 5.0 V/5.0 A Converter
160 k
20.25 k
FF
10
GATE
IRF634
330 pF
0.01 µF
SS
ISENSE
LGND
PGND
62
10
470 pF
0.1 µF
5.1 k
2.0 k
5.6 k
180
1.0 k
TL431
2.0 k
150
4700 pF
MOC81025
1.0 k
CS51221
ABSOLUTE MAXIMUM RATINGS*
Rating
Operating Junction Temperature, TJ
Lead Temperature Soldering:
Reflow: (SMD styles only) (Note 1.)
Storage Temperature Range, TS
ESD (Human Body Model)
Value
Unit
Internally
Limited
–
230 peak
°C
–65 to +150
°C
2.0
kV
1. 60 second maximum above 183°C.
*The maximum package power dissipation must be observed.
ABSOLUTE MAXIMUM RATINGS
Pin Name
Pin Symbol
VMAX
VMIN
ISOURCE
ISINK
Gate Drive Output
GATE
15 V
–0.3 V
1.0 A Peak, 200 mA DC
1.0 A Peak, 200 mA DC
Current Sense Input
ISENSE
6.0 V
–0.3 V
1.0 mA
1.0 mA
Timing Resistor/Capacitor
RTCT
6.0 V
–0.3 V
1.0 mA
10 mA
Feed Forward
FF
6.0 V
–0.3 V
1.0 mA
25 mA
Error Amp Output
COMP
6.0 V
–0.3 V
10 mA
20 mA
Feedback Voltage
VFB
6.0 V
–0.3 V
1.0 mA
1.0 mA
Sync Input
SYNC
6.0 V
–0.3 V
10 mA
10 mA
Undervoltage
UV
6.0 V
–0.3 V
1.0 mA
1.0 mA
Overvoltage
OV
6.0 V
–0.3 V
1.0 mA
1.0 mA
Current Set
ISET
6.0 V
–0.3 V
1.0 mA
1.0 mA
Soft Start
SS
6.0 V
–0.3 V
1.0 mA
10 mA
Logic Section Supply
VCC
15 V
–0.3 V
10 mA
50 mA
Power Section Supply
VC
15 V
–0.3 V
10 mA
1.0 A Peak, 200 mA DC
Reference Voltage
VREF
6.0 V
–0.3 V
lnternally Limited
10 mA
Power Ground
PGND
N/A
N/A
1.0 A Peak, 200 mA DC
N/A
Logic Ground
LGND
N/A
N/A
N/A
N/A
ELECTRICAL CHARACTERISTICS (–40°C < TA < 85°C; –40°C < TJ < 125°C; 3.0 V < VC < 15 V; 4.7 V < VCC < 15 V;
RT = 12 k; CT = 390 pF; unless otherwise specified.)
Test Conditions
Min
Typ
Max
Unit
Start Threshold
–
4.4
4.6
4.7
V
Stop Threshold
–
3.2
3.8
4.1
V
400
850
1400
mV
–
38
75
µA
–
9.5
14
mA
Characteristic
Start/Stop Voltages
Hysteresis
Start–Stop
ICC @ Startup
VCC < UVL Start Threshold
Supply Current
ICC Operating
–
IC Operating
1.0 nF Load on GATE
–
12
18
mA
IC Operating
No Switching
–
2.0
4.0
mA
3.2
3.3
3.4
V
Reference Voltage
Total Accuracy
0 mA < IREF < 2.0 mA
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CS51221
ELECTRICAL CHARACTERISTICS (continued) (–40°C < TA < 85°C; –40°C < TJ < 125°C; 3.0 V < VC < 15 V; 4.7 V < VCC < 15 V;
RT = 12 k; CT = 390 pF; unless otherwise specified.)
Characteristic
Test Conditions
Min
Typ
Max
Unit
–
–
6.0
20
mV
Reference Voltage (continued)
Line Regulation
Load Regulation
0 mA < IREF < 2.0 mA
–
6.0
15
mV
Noise Voltage
10 Hz < F < 10 kHz. Note 2.
–
50
–
µV
Op Life Shift
T = 1000 Hrs. Note 2.
–
4.0
20
mV
Fault Voltage
–
2.8
2.95
3.1
V
VREF(OK) Voltage
–
2.9
3.05
3.2
V
VREF(OK) Hysteresis
–
30
100
150
mV
Current Limit
–
2.0
40
100
mA
1.234
1.263
1.285
V
Error Amp
Reference Voltage
VFB = COMP
VFB Input Current
VFB = 1.2 V
–
1.3
2.0
µA
Open Loop Gain
Note 2.
60
–
–
dB
Unity Gain Bandwidth
Note 2.
1.5
–
–
MHz
COMP Sink Current
COMP = 1.4 V, VFB = 1.45 V
3.0
12
32
mA
COMP Source Current
COMP = 1.4 V, VFB = 1.15 V
1.0
1.6
2.0
mA
COMP High Voltage
VFB = 1.15 V
2.8
3.1
3.4
V
COMP Low Voltage
VFB = 1.45 V
75
125
300
mV
PSRR
Freq = 120 Hz. Note 2.
60
85
–
dB
SS Clamp, VCOMP
SS = 1.4 V, VFB = 0 V, ISET = 2.0 V
1.3
1.4
1.5
V
COMP Max Clamp
Note 2.
1.7
1.8
1.9
V
–
260
273
320
kHz
–
–
1.0
2.0
%
–
8.0
–
%
1.0
–
–
MHz
80
85
90
%
1.94
2.0
2.06
V
0.9
0.95
1.0
V
0.85
1.0
1.15
V
–
0.85
1.0
1.15
mA
Input Threshold
–
0.9
1.4
1.8
V
Output Pulse Width
–
200
320
450
ns
2.1
2.5
2.8
V
35
70
140
kΩ
Oscillator
Frequency Accuracy
Voltage Stability
Temperature Stability
–40°C < TJ < 125°C. Note 2.
Max Frequency
Note 2.
Duty Cycle
Peak Voltage
–
Note 2.
Valley Clamp Voltage
Valley Voltage
–
Note 2.
Discharge Current
Synchronization
Output High Voltage
100 µA Load
Input Resistance
–
SYNC to Drive Delay
Time from SYNC to GATE Shutdown
100
140
180
ns
Output Drive Current
RSYNC = 1.0 Ω
1.0
1.5
2.25
mA
2. Guaranteed by design, not 100% tested in production.
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CS51221
ELECTRICAL CHARACTERISTICS (continued) (–40°C < TA < 85°C; –40°C < TJ < 125°C; 3.0 V < VC < 15 V; 4.7 V < VCC < 15 V;
RT = 12 k; CT = 390 pF; unless otherwise specified.)
Characteristic
Test Conditions
Min
Typ
Max
Unit
GATE Driver
High Saturation Voltage
VC – GATE, VC = 10 V, ISOURCE = 200 mA
–
1.5
2.0
V
Low Saturation Voltage
GATE – PGND, ISINK = 200 mA
–
1.2
1.5
V
11
13.5
16
V
High Voltage Clamp
–
Output Current
1.0 nF Load. Note 3.
–
1.0
1.25
A
Output UVL Leakage
GATE = 0 V
–
1.0
50
µA
Rise Time
1.0 nF Load, VC = 20 V, 1.0 V < GATE < 9.0 V
–
60
100
ns
Fall Time
1.0 nF Load, VC = 20 V, 9.0 V < GATE < 1.0 V
–
25
50
ns
Max Gate Voltage During UVL/Sleep
IGATE = 500 µA
0.4
0.7
1.0
V
–
0.3
0.7
V
2.0
16
30
mA
50
75
125
ns
0.475
0.5
0.525
V
50
90
125
ns
Feed Forward (FF)
Discharge Voltage
IFF = 2.0 mA
Discharge Current
FF = 1.0 V
FF to GATE Delay
–
Overcurrent Protection
Overcurrent Threshold
ISET = 0.5 V, Ramp ISENSE
ISENSE to GATE Delay
–
External Voltage Monitors
Overvoltage Threshold
OV Increasing
1.9
2.0
2.1
V
Overvoltage Hysteresis Current
OV = 2.15 V
10
12.5
15
µA
Undervoltage Threshold
UV Increasing
0.95
1.0
1.05
V
25
75
125
mV
Undervoltage Hysteresis
–
Soft Start (SS)
Charge Current
SS = 2.0 V
40
50
70
µA
Discharge Current
SS = 2.0 V
4.0
5.0
7.0
µA
Charge Voltage
–
2.8
3.0
3.4
V
Discharge Voltage
–
0.25
0.3
0.35
V
1.15
1.25
1.35
V
–
0.1
0.2
V
50
150
250
ns
Soft Start Clamp Offset
FF = 1.25 V
Soft Start Fault Voltage
OV = 2.15 V or LV = 0.85 V
Blanking
Blanking Time
–
SS Blanking Disable Threshold
VFB < 1.0
2.8
3.0
3.3
V
COMP Blanking Disable Threshold
VFB < 1.0, SS > 3.0 V
2.8
3.0
3.3
V
Thermal Shutdown
Note 3.
125
150
180
°C
Thermal Hysteresis
Note 3.
5.0
10
15
°C
Thermal Shutdown
3. Guaranteed by design, not 100% tested in production.
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5
CS51221
PACKAGE PIN DESCRIPTION
PACKAGE PIN #
SO–16
PIN SYMBOL
FUNCTION
1
GATE
External power switch driver with 1.0 A peak capability. Rail
to rail output occurs when the capacitive load is between
470 pF and 10 nF.
2
ISENSE
Current sense comparator input.
3
SYNC
Bidirectional synchronization. Locks to highest frequency.
4
FF
PWM ramp.
5
UV
Undervoltage protection monitor.
6
OV
Overvoltage protection monitor.
7
RTCT
Timing resistor RT and capacitor CT determine oscillator
frequency and maximum duty cycle, DMAX.
8
ISET
Voltage at this pin sets pulse–by–pulse overcurrent threshold.
9
VFB
Feedback voltage input. Connected to the error amplifier
inverting input.
10
COMP
11
SS
12
LGND
Logic ground.
13
VREF
3.3 V reference voltage output. Decoupling capacitor can be
selected from 0.01 µF to 10 µF.
14
VCC
Logic supply voltage.
15
PGND
16
VC
Error amplifier output.
Charging external capacitor restricts error amplifier output
voltage during the power up or fault conditions.
Output power stage ground.
Output power stage supply voltage.
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CS51221
VCC
2.0 mA (maximum load current)
VREF
3.3 V
+
UVL
–
+
–
ENABLE
VREF = 3.3 V
–
Thermal
Shutdown
VREF OK
+
3.1 V
UV Lockout
Start/Stop
Low Sat
Gate Driver
S
Q
GATE
G1
SYNC
RTCT
OSC
VBG
(1.263 V)
VFB
G2
2.0 V to 1.0 V Trip Points
R
13.5 V
Q
PGND
3.0 V
+
EAMP
–
VC
–
Max Duty Cycle
+
(Sat Sense)
VREF
50 µA
SS to 1.8 V Max
+
–
+
Soft Start Clamp
–
COMP
LGND
PWM
Comp
ON
FF
FF Discharge
VO Off
G4
Latching
Discharge
G3
OV Monitor
Max SS
Det
+
ISET
DISABLE
–
150 ns
Blank
ILIM
3.0 V
–
SS
5.0 µA
+
OV
–
2.0 V
+
(Sat Sense)
UV Monitor
–
UV
ISENSE
+
Figure 2. Block Diagram
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1.0 V
CS51221
APPLICATION INFORMATION
THEORY OF OPERATION
of the volt–second clamp are presented in the Design
Guidelines section.
Feed Forward Voltage Mode Control
VOUT
In conventional voltage mode control, the ramp signal has
fixed rising and falling slope. The feedback signal is derived
solely from the output voltage. Consequently, voltage mode
control has inferior line regulation and audio susceptibility.
Feed forward voltage mode control derives the ramp
signal from the input line, as shown in Figure 3. Therefore,
the ramp of the slope varies with the input voltage. At the
start of each switch cycle, the capacitor connected to the FF
pin is charged through a resistor connected to the input
voltage. Meanwhile, the Gate output is turned on to drive an
external power switching device. When the FF pin voltage
reaches the error amplifier output VCOMP, the PWM
comparator turns off the Gate, which in turn opens the
external switch. Simultaneously, the FF capacitor is quickly
discharged to 0.3 V.
Overall, the dynamics of the duty cycle are controlled by
both input and output voltages. As illustrated in Figure 4,
with a fixed input voltage the output voltage is regulated
solely by the error amplifier. For example, an elevated
output voltage reduces VCOMP which in turn causes duty
cycle to decrease. However, if the input voltage varies, the
slope of the ramp signal will react immediately which
provides a much improved line transient response. As an
example shown in Figure 5, when the input voltage goes up,
the rising edge of the ramp signal increases which reduces
duty cycle to counteract the change.
VCOMP
FF
VIN
RTCT
GATE
Figure 4. Pulse Width Modulated by Output
Current with Constant Input Voltage
VIN
VCOMP
FF
IOUT
VIN
VOUT
Power Stage
RTCT
GATE
R
GATE
Latch & Driver
Feedback
Network
Figure 5. Pulse Width Modulated by Input Voltage
with Constant Output Current
PWM
FF
COMP
FB
–
Powering the IC & UVL
+
C
Error Amplifier
The Under Voltage Lockout (UVL) comparator has two
voltage references; the start and stop thresholds. During
power–up, the UVL comparator disables VREF (which
in–turn disables the entire IC) until the controller reaches its
VCC start threshold. During power–down, the UVL
comparator allows the controller to operate until the VCC
stop threshold is reached. The CS51221 requires only 50 µA
during startup. The output stage is held at a low impedance
state in lock out mode.
During power up and fault conditions, the soft–start
clamps the Comp pin voltage and limits the duty cycle. The
power up transition tends to generate temporary duty cycles
+
–
Figure 3. Feed Forward Voltage Mode Control
The feed forward feature can also be employed to provide
a volt–second clamp, which limits the maximum product of
input voltage and turn on time. This clamp is used in circuits,
such as Forward and Flyback converter, to prevent the
transformer from saturating. Calculations used in the design
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CS51221
sets the threshold for maximum current. As shown in Figure
7, when the ISENSE pin voltage exceeds the ISET voltage, the
current limit comparator will reset the GATE latch flip–flop
to terminate the GATE pulse.
much greater than the steady state value due to the low
output voltage. Consequently, excessive current stresses
often take place in the system. Soft Start technique alleviates
this problem by gradually releasing the clamp on the duty
cycle to eliminate the in–rush current. The duration of the
Soft Start can be programmed through a capacitance
connected to the SS pin. The constant charging current to the
SS pin is 50 µA (typ).
The VREF (ok) comparator monitors the 3.3 V VREF
output and latches a fault condition if VREF falls below 3.1 V.
The fault condition may also be triggered when the OV pin
voltage rises above 2.0 V or the UV pin voltage falls below
1.0 V. The under–voltage comparator has a built–in
hysteresis of 75 mV (typ). The hysteresis for the OV
comparator is programmable through a resistor connected to
the OV pin. When an OV condition is detected, the
over–voltage hysteresis current of 12.5 µA (typ) is sourced
from the pin.
In Figure 6, the fault condition is triggered by pulling the
UV pin to the ground. Immediately, the SS capacitor is
discharged with 5.0 µA of current (typ) and the GATE output
is disabled until the SS voltage reaches the discharge voltage
of 0.3 V (typ). The IC starts the Soft Start transition again if
the fault condition has recovered as shown in Figure 6.
However, if the fault condition persists, the SS voltage will
stay at 0.1 V until the removal of the fault condition.
Figure 7. The GATE Output Is Terminated When
the ISENSE Pin Voltage Reaches the Threshold Set
By the ISET Pin. CH2: ISENSE Pin, CH4: ISET Pin,
CH3: GATE Pin
The current sense signal is prone to leading edge spikes
caused by the switching transition. A RC low–pass filter is
usually applied to the current signals to avoid premature
triggering. However, the low pass filter will inevitably
change the shape of the current pulse and also add cost. The
CS51221 uses leading edge blanking circuitry that blocks
out the first 150 ns (typ) of each current pulse. This removes
the leading edge spikes without altering the current
waveform. The blanking is disabled during Soft Start and
when the VCOMP is saturated high so that the minimum
on–time of the controller does not have the additional
blanking period. The max SS detect comparator keeps the
blanking function disabled until SS charges fully. The output
of the max Duty Cycle detector goes high when the error
amplifier output gets saturated high, indicating that the
output voltage has fallen well below its regulation point and
the power supply may be under load stress.
Figure 6. The Fault Condition Is Triggered when
the UV Pin Voltage Falls Below 1.0 V. The Soft
Start Capacitor Is Discharged and the GATE
Output Is Disabled. CH2: Envelop of GATE Output,
CH3: SS Pin with 0.01 F Capacitor, CH4: UV Pin
Oscillator and Synchronization
The switching frequency is programmable through a RC
network connected to the RTCT Pin. As shown in Figure 8,
when the RTCT pin reaches 2.0 V, the capacitor is discharged
by a 1.0 mA current source and the Gate signal is disabled.
When the RTCT pin decreases to 1.0 V, the Gate output is
turned on and the discharge current is removed to let the
RTCT pin ramp up. This begins a new switching cycle. The
CT charging time over the switch period sets the maximum
duty cycle clamp which is programmable through the RT
value as shown in the Design Guidelines. At the beginning
Current Sense and Over Current Protection
The current can be monitored by the ISENSE pin to achieve
pulse by pulse current limit. Various techniques, such as a
using current sense resistor or current transformer, can be
adopted to derive current signals. The voltage of the ISET pin
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CS51221
of each switching cycle, the SYNC pin generates a 2.5 V,
320 nS (typ) pulse. This pulse can be utilized to synchronize
other power supplies.
Figure 9. Operation with External Sync.
CH2: SYNC Pin, CH3: GATE Pin, CH4: RTCT Pin
Figure 8. The SYNC Pin Generates a Sync Pulse at
the Beginning of Each Switching Cycle.
CH2: GATE Pin, CH3: RTCT, CH4: SYNC Pin
DESIGN GUIDELINES
Switch Frequency and Maximum Duty Cycle
Calculations
An external pulse signal can feed to the bidirectional
SYNC pin to synchronize the switch frequency. For reliable
operation, the sync frequency should be approximately 20%
higher than free running IC frequency. As show in Figure 9,
when the SYNC pin is triggered by an incoming signal, the
IC immediately discharges CT. The GATE signal is turned
on once the RTCT pin reaches the valley voltage. Because of
the steep falling edge, this valley voltage falls below the
regular 1.0 V threshold. However, the RTCT pin voltage is
then quickly raised by a clamp. When the RTCT pin reaches
the 0.95 V (typ) Valley Clamp Voltage, the clamp is
disconnected after a brief delay and CT is charged through
RT.
Oscillator timing capacitor, CT, is charged by VREF
through RT and discharged by an internal current source.
During the discharge time, the internal clock signal sets the
Gate output to the low state, thus providing a user selectable
maximum duty cycle clamp. Charge and discharge times are
determined by following general formulas;
VVALLEY)
(V(VREF
REF VPEAK)
tC RTCT ln
(VREF VPEAK IdRT)
td RTCT ln
(VREF VVALLEY IdRT)
where:
tC = charging time;
td = discharging time;
VVALLEY = valley voltage of the oscillator;
VPEAK = peak voltage of the oscillator.
Substituting in typical values for the parameters in the
above formulas, VREF = 3.3 V, VVALLEY = 1.0 V, VPEAK =
2.0 V, Id = 1.0 mA:
tC 0.57RTCT
1.3 0.001RT
td RTCT ln
2.3 0.001RT
D max 0.57
T
0.57 In1.30.001R
2.30.001R
T
It is noticed from the equation that for the oscillator to
function properly, RT has to be greater than 2.3 k.
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CS51221
800
1.00
0.95
700
0.90
RT = 5.0 K
Duty Cycle (%)
Frequency (kHz)
600
500
400
RT = 10 K
300
200
0.85
0.80
0.75
0.70
0.65
0.60
100
0
0.0001
0.55
RT = 50 K
0.50
1000
0.01
0.001
RT (Ω)
Figure 10. Typical Performance Characteristics,
Oscillator Frequency vs. CT
Figure 11. Typical Performance Characteristics,
Oscillator Duty Cycle vs. RT
R1 R2
V 1.27 R2 OUT
Select RC for Feed Forward Ramp
If the line voltage is much greater than the FF pin Peak
Voltage, the charge current can be treated as a constant and
is equal to VIN/R. Therefore, the volt–second value is
determined by:
where ∇ is the correction factor due to the existence of the
FB pin input current Ier.
(Ri R1R2)Ier
VIN TON (VCOMP VFF(d)) R C
Ri = DC resistance between the FB pin and the voltage
divider output.
Ier = VFB input current, 1.3 µA typical.
where:
VCOMP = COMP pin voltage;
VFF(d) = FF pin discharge voltage.
As shown in the equation, the volt–second clamp is set by
the VCOMP clamp voltage which is equal to 1.8 V. In
Forward or Flyback circuits, the volt–second clamp value is
designed to prevent transformers from saturation.
In a buck or forward converter, volt–second is equal to
V
TS
VIN TON OUTn
1000000
100000
10000
CT (µF)
VOUT
Ier
–
R1
FB
Ri
COMP
+
+
–
n = transformer turns ratio, which is a constant determined
by the regulated output voltage, switching period and
transformer turns ration (use 1.0 for buck converter). It is
interesting to notice from the aforementioned two equations
that during steady state, VCOMP doesn’t change for input
voltage variations. This intuitively explains why FF voltage
mode control has superior line regulation and line transient
response. Knowing the nominal value of VIN and TON, one
can also select the value of RC to place VCOMP at the center
of its dynamic range.
1.27
R2
Figure 12. The Design of Feedback Voltage Divider
Has to Consider the Error Amplifier Input Current
Design Voltage Dividers for OV and UV Detection
In Figure 13, the voltage divider uses three resistors in
series to set OV and UV threshold seen from the input
voltage. The values of the resistors can be calculated from
the following three equations, where the third equation is
derived from OV hysteresis requirement.
Select Feedback Voltage Divider
As shown in Figure 12, the voltage divider output feeds to
the FB pin, which connects to the inverting input of the error
amplifier. The non–inverting input of the error amplifier is
connected to a 1.27 V (typ) reference voltage. The FB pin
has an input current which has to be considered for accurate
DC outputs. The following equation can be used to calculate
the R1 and R2 value
VIN(LOW) http://onsemi.com
11
R3 1.0 V
R2 R2
R3 R1
(A)
CS51221
VIN(HIGH) R3
R2 R3
2.0 V
R1
12.5 A (R1 R2) VHYST
VIN(LOW). Otherwise, two voltage dividers have to be used
to program OV and UV separately.
(B)
(C)
R1
R2
R3
VIN
where:
VIN(LOW), VIN(HIGH) = input voltage OV and UV
threshold;
VHYST = OV hysteresis seen at VIN
It is self–evident from equation A and B that to use this
design, VIN(HIGH) has to be two times greater than
VUV
VOV
Figure 13. OV/UV Monitor Divider
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12
CS51221
PACKAGE DIMENSIONS
SO–16
D SUFFIX
CASE 751B–05
ISSUE J
–A–
16
NOTES:
1. DIMENSIONING AND TOLERANCING PER ANSI
Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSIONS A AND B DO NOT INCLUDE
MOLD PROTRUSION.
4. MAXIMUM MOLD PROTRUSION 0.15 (0.006)
PER SIDE.
5. DIMENSION D DOES NOT INCLUDE DAMBAR
PROTRUSION. ALLOWABLE DAMBAR
PROTRUSION SHALL BE 0.127 (0.005) TOTAL
IN EXCESS OF THE D DIMENSION AT
MAXIMUM MATERIAL CONDITION.
9
–B–
1
P
8 PL
0.25 (0.010)
8
M
B
S
G
R
K
F
X 45 C
–T–
SEATING
PLANE
J
M
D
16 PL
0.25 (0.010)
M
T B
S
A
S
DIM
A
B
C
D
F
G
J
K
M
P
R
MILLIMETERS
MIN
MAX
9.80
10.00
3.80
4.00
1.35
1.75
0.35
0.49
0.40
1.25
1.27 BSC
0.19
0.25
0.10
0.25
0
7
5.80
6.20
0.25
0.50
PACKAGE THERMAL DATA
Parameter
SO–16
Unit
RΘJC
Typical
28
°C/W
RΘJA
Typical
115
°C/W
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13
INCHES
MIN
MAX
0.386
0.393
0.150
0.157
0.054
0.068
0.014
0.019
0.016
0.049
0.050 BSC
0.008
0.009
0.004
0.009
0
7
0.229
0.244
0.010
0.019
CS51221
Notes
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14
CS51221
Notes
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15
CS51221
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CS51221/D