UC3845D ON Semiconductor, UC3845D Datasheet - Page 10

• Manufacturer Part Number: UC3845D
• Description: Other Power Management 52kHz 1A Current PWM
• Manufacturer: ON Semiconductor
• Type: High Performance Current Mode Controllersr

Specifications of UC3845D

• Output Voltage Range: 4.9 V to 5.1 V
• Input Voltage Range: 30 V
• Operating Temperature Range: 0 C to + 70 C
• Mounting Style: SMD/SMT
• Package / Case: SOIC-14
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

V
Clamp
I
C
pk(max)
R2
R1
t
Softstart
Figure 22. Adjustable Buffered Reduction of
8(14)
R2
R1
4(7)
2(3)
1(1)
≈
8(14)
Figure 20. Adjustable Reduction of Clamp Level
4(7)
R
R
2(3)
1(1)
= − In
MPSA63
1.67
2
1
V
V
Clamp
Clamp
R
+ 1
S
Clamp Level with Soft−Start
+
−
EA
R
R
+
−
+ 0.33 x 10
1 −
+
Where: 0 ≤ V
EA
R
R
R
R
OSC
1.67
2
1
1.0mA
Bias
+
3V
OSC
+ 1
1.0mA
Bias
V
2R
Clamp
−3
C
R
+ 0.33 x 10
2R
Clamp
R
5(9)
V
C
≤ 1.0 V
Clamp
R
5(9)
−
R
+
V
1.0V
1
5.0V
1
R
Clamp
−3
R
+ R
1
−
+
R
1
−
+
1.0V
5.0V
+
−
+ R
2
ref
2
R
−
+
−
+
R
2
5.0V
+
−
R
2
ref
1
1
Comp/Latch
+ R
−
+
Comp/Latch
S
R
+
−
R
ref
T
Q
2
S
R
2
Comp/Latch
T
Figure 24. Current Waveform Spike Suppression
Q
I
S
R
pk(max)
+
T
−
Q
Where: 0 ≤ V
+
−
+
7(12)
−
≈
+
−
+
7(12)
−
V
+
Clamp
7(12)
−
R
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Clamp
V
S
CC
7(11)
6(10)
5(8)
3(5)
V
CC
7(11)
6(10)
5(8)
3(5)
≤ 1.0 V
V
CC
7(11)
6(10)
5(8)
3(5)
10
Q1
Q1
R
V
R
V
S
in
S
in
C
R
Virtually lossless current sensing can be achieved with the implement of a SENSEFET
power switch. For proper operation during over current conditions, a reduction of the
I
pk(max)
−
+
5.0V
Figure 23. Current Sensing Power MOSFET
Q1
R
V
clamp level must be implemented. Refer to Figures 20 and 22.
−
+
+
−
S
in
ref
C
Comp/Latch
S
R
The addition of the RC filter will eliminate
instability caused by the leading edge spike on
the current waveform.
T
Q
Control CIrcuitry
8(14)
4(7)
2(3)
1(1)
t
+
To Pin (9)
−
Soft−Start
Ground:
Figure 21. Soft−Start Circuit
+
(12)
−
 3600C in mF
+
−
EA
R
R
V
CC
+
(11)
(10)
(8)
(5)
OSC
1.0mA
Bias
G
1/4 W
R
2R
M
S
D
R
SENSEFET
V
in
To Input Source
K
Power Ground
5(9)
S
If: SENSEFET = MTP10N10M
Return
V
−
Pin
+
1.0V
5.0V
Then: V
5 ≈
−
+
+
−
ref
R
r
pin
S
DM(on)
R
S
R
T
I
S
5 = 0.075 I
pk
Q
= 200
r
DS(on)
+ R
S
pk