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 50mA startup current, over and undervoltage protection, and
bidirectional synchronization. The CS51221 is available in a 16 lead
SOIC narrow surface mount package.
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SOIC−16
D SUFFIX
CASE 751B
16
1
TSSOP−16
DTB SUFFIX
CASE 948F
Features
16
1
PIN CONNECTIONS AND
MARKING DIAGRAM
1
16
GATE
ISENSE
SYNC
FF
UV
OV
RTCT
ISET
1
VC
PGND
VCC
VREF
LGND
SS
COMP
VFB
16
CS51
221
ALYWG
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
Pb−Free Packages are Available*
CS51221
AWLYWW
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
CS51221= Specific Device Code
A
= Assembly Location
WL, L = Wafer Lot
YY, Y
= Year
WW, W = Work Week
G
= Pb−Free Package
ORDERING INFORMATION
Device
Package
Shipping†
CS51221ED16
SOIC−16
48 Units / Rail
CS51221ED16G
SOIC−16 48 Units / Rail
(Pb−Free)
SOIC−16 2500 Tape & Reel
CS51221EDR16
CS51221EDR16G
CS51221EDTB16G
SOIC−16 2500 Tape & Reel
(Pb−Free)
TSSOP−16 96 Units / Rail
(Pb−Free)
CS51221EDTB16R2G TSSOP−16 2500 Tape & Reel
(Pb−Free)
*For additional information on our Pb−Free strategy and soldering details, please
download the ON Semiconductor Soldering and Mounting Techniques
Reference Manual, SOLDERRM/D.
© Semiconductor Components Industries, LLC, 2005
September, 2005 − Rev. 8
1
†For information on tape and reel specifications,
including part orientation and tape sizes, please
refer to our Tape and Reel Packaging Specifications
Brochure, BRD8011/D.
Publication Order Number:
CS51221/D
1.0 mF
10 k
330 pF
200
2200 pF
0.01 mF
0.22 mF
11 V
51 k
VIN (36 V to 72 V)
2
GATE
ISENSE
PGND
SS
LGND
FF
ISET
OV
UV
VCC
10
22 mF 18 V
SYNC
RTCT
VFB
COMP
VREF
VC
FZT688
CS51221
Figure 1. Application Diagram, 36 V−72 V to 5.0 V/5.0 A Converter
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150
470 pF
5.6 k
10
13 k
4.3 k
D11
BAS21
10 k
T3
100:1
180
100 pF
MOC81025
62
IRF634
100
1.0 mF
510 k
1.0 k
20.25 k
24.3 k
4700 pF
0.1 mF
160 k
BAS21
10
680 pF
10
TL431
0.1 mF
5.1 k
2.0 k
100 mF
2.0 k
MBRB2545CT
D13
V33MLA1206A23
1.0 k
T1
4:1
T2
2:5
SGND
VOUT
(5.0 V/5.0 A)
CS51221
CS51221
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
Maximum ratings are those values beyond which device damage can occur. Maximum ratings applied to the device are individual stress limit
values (not normal operating conditions) and are not valid simultaneously. If these limits are exceeded, device functional operation is not implied,
damage may occur and reliability may be affected.
1. 60 second maximum above 183°C.
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.)
Characteristic
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
mA
−
9.5
14
mA
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
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CS51221
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.)
Characteristic
Test Conditions
Min
Typ
Max
Unit
3.2
3.3
3.4
V
Reference Voltage
Total Accuracy
0 mA < IREF < 2.0 mA
Line Regulation
−
6.0
20
mV
Load Regulation
0 mA < IREF < 2.0 mA
−
−
6.0
15
mV
Noise Voltage
10 Hz < F < 10 kHz. Note 2
−
50
−
mV
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
mA
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
Oscillator
Frequency Accuracy
−
260
273
320
kHz
Voltage Stability
−
−
1.0
2.0
%
−
8.0
−
%
1.0
−
−
MHz
80
85
90
%
1.94
2.0
2.06
V
Temperature Stability
−40°C < TJ < 125°C. (Note 2)
Max Frequency
Note 2
Duty Cycle
Peak Voltage
−
Note 2
Valley Clamp Voltage
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
kW
Valley Voltage
−
Note 2
Discharge Current
Synchronization
Output High Voltage
100 mA Load
Input Resistance
−
SYNC to Drive Delay
Time from SYNC to GATE Shutdown
100
140
180
ns
Output Drive Current
RSYNC = 1.0 W
1.0
1.5
2.25
mA
2. Guaranteed by design, not 100% tested in production.
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CS51221
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.)
Characteristic
Test Conditions
Min
Typ
Max
Unit
−
1.5
2.0
V
Gate Driver
High Saturation Voltage
VC − GATE, VC = 10 V, ISOURCE = 200 mA
Low Saturation Voltage
GATE − PGND, ISINK = 200 mA
High Voltage Clamp
−
−
1.2
1.5
V
11
13.5
16
V
Output Current
1.0 nF Load. Note 3
−
1.0
1.25
A
Output UVL Leakage
GATE = 0 V
−
1.0
50
mA
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 mA
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
mA
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
mA
Discharge Current
SS = 2.0 V
4.0
5.0
7.0
mA
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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CS51221
PACKAGE PIN DESCRIPTION
Package
Pin #
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 mF to 10 mF.
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 mA
SS to 1.8 V Max
+
+
−
COMP
Soft−Start Clamp
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 mA
+
OV
−
2.0 V
+
(Sat Sense)
UV Monitor
−
UV
ISENSE
+
Figure 2. Block Diagram
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7
1.0 V
CS51221
APPLICATION INFORMATION
THEORY OF OPERATION
VOUT
Feed Forward Voltage Mode Control
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.
VIN
VCOMP
FF
VIN
RTCT
GATE
Figure 4. Pulse Width Modulated by Output
Current with Constant Input Voltage
VIN
VCOMP
FF
IOUT
RTCT
VOUT
Power Stage
GATE
GATE
R
Figure 5. Pulse Width Modulated by Input Voltage
with Constant Output Current
Latch & Driver
Feedback
Network
Powering the IC & UVL
PWM
The Undervoltage 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 mA
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
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
FF
COMP
FB
−
+
C
Error Amplifier
+
−
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
of the volt−second clamp are presented in the Design
Guidelines section.
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8
CS51221
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 mA (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 undervoltage 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 overvoltage
hysteresis current of 12.5 mA (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 mA 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 underload 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 mF 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
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.
Current Sense and Overcurrent 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
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.
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9
CS51221
DESIGN GUIDELINES
Switch Frequency and Maximum Duty Cycle
Calculations
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)
ǒ(V(VREF
Ǔ
REF * VPEAK)
tC + RTCT ln
ǒ
(VREF * VPEAK * IdRT)
td + RTCT ln
(VREF * VVALLEY * IdRT)
Ǔ
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:
Figure 8. The SYNC Pin Generates a Sync Pulse at
the Beginning of Each Switching Cycle.
CH2: GATE Pin, CH3: RTCT, CH4: SYNC Pin
tC + 0.57RTCT
ǒ
Ǔ
1.3 * 0.001RT
td + RTCT ln
2.3 * 0.001RT
D max +
0.57
TǓ
0.57 ) Inǒ1.3*0.001R
2.3*0.001R
T
It is noticed from the equation that for the oscillator to
function properly, RT has to be greater than 2.3 k.
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:
VIN
Figure 9. Operation with External Sync.
CH2: SYNC Pin, CH3: GATE Pin, CH4: RTCT Pin
TON + (VCOMP * VFF(d))
R
C
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
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.
VIN
TON +
ǒVOUTn
TS
Ǔ
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
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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 (W)
Figure 10. Typical Performance Characteristics,
Oscillator Frequency vs. CT
Figure 11. Typical Performance Characteristics,
Oscillator Duty Cycle vs. RT
12.5 mA
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.
(R1 ) R2) + VHYST
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
VOUT
Ier
Ǔ
R1
R2
V
+ 1.27 * ʼn
R1 ) R2 OUT
−
where ∇ is the correction factor due to the existence of the
FB pin input current Ier.
+
+
−
Ri = DC resistance between the FB pin and the voltage
divider output.
Ier = VFB input current, 1.3 mA typical.
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.
(A)
VIN(HIGH)
R3
ǒR2 ) R3
Ǔ + 2.0 V
) R1
(B)
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
) R3 Ǔ + 1.0 V
ǒR2 R2
) R3 ) R1
FB
Ri
COMP
ʼn + (Ri ) R1ńńR2)Ier
VIN(LOW)
(C)
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
VIN(LOW). Otherwise, two voltage dividers have to be used
to program OV and UV separately.
Select Feedback Voltage Divider
ǒ
1000000
100000
10000
CT (mF)
R1
R2
R3
VIN
VUV
VOV
Figure 13. OV/UV Monitor Divider
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11
CS51221
PACKAGE DIMENSIONS
SOIC−16
D SUFFIX
CASE 751B−05
ISSUE J
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.
−A−
16
9
1
8
−B−
P
8 PL
0.25 (0.010)
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
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
S
PACKAGE THERMAL DATA
Parameter
SOIC−16
Unit
RqJC
Typical
28
°C/W
RqJA
Typical
115
°C/W
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12
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
PACKAGE DIMENSIONS
TSSOP−16
CASE 948F−01
ISSUE A
NOTES:
1. DIMENSIONING AND TOLERANCING PER
ANSI Y14.5M, 1982.
2. CONTROLLING DIMENSION: MILLIMETER.
3. DIMENSION A DOES NOT INCLUDE MOLD
FLASH. PROTRUSIONS OR GATE BURRS.
MOLD FLASH OR GATE BURRS SHALL NOT
EXCEED 0.15 (0.006) PER SIDE.
4. DIMENSION B DOES NOT INCLUDE
INTERLEAD FLASH OR PROTRUSION.
INTERLEAD FLASH OR PROTRUSION SHALL
NOT EXCEED 0.25 (0.010) PER SIDE.
5. DIMENSION K DOES NOT INCLUDE
DAMBAR PROTRUSION. ALLOWABLE
DAMBAR PROTRUSION SHALL BE 0.08
(0.003) TOTAL IN EXCESS OF THE K
DIMENSION AT MAXIMUM MATERIAL
CONDITION.
6. TERMINAL NUMBERS ARE SHOWN FOR
REFERENCE ONLY.
7. DIMENSION A AND B ARE TO BE
DETERMINED AT DATUM PLANE −W−.
16X K REF
0.10 (0.004)
0.15 (0.006) T U
M
T U
V
S
S
S
2X
L/2
16
9
K
ÇÇÇ
ÉÉ
ÇÇÇ
ÉÉ
K1
B
−U−
L
J1
PIN 1
IDENT.
SECTION N−N
8
1
J
N
0.15 (0.006) T U
S
DIM
A
B
C
D
F
G
H
J
J1
K
K1
L
M
0.25 (0.010)
A
−V−
M
N
F
DETAIL E
−W−
C
0.10 (0.004)
−T− SEATING
PLANE
H
D
DETAIL E
G
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13
MILLIMETERS
MIN
MAX
4.90
5.10
4.30
4.50
−−−
1.20
0.05
0.15
0.50
0.75
0.65 BSC
0.18
0.28
0.09
0.20
0.09
0.16
0.19
0.30
0.19
0.25
6.40 BSC
0_
8_
INCHES
MIN
MAX
0.193 0.200
0.169 0.177
−−− 0.047
0.002 0.006
0.020 0.030
0.026 BSC
0.007
0.011
0.004 0.008
0.004 0.006
0.007 0.012
0.007 0.010
0.252 BSC
0_
8_
CS51221
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CS51221D