Converting SC4525C/D to SC4525E/F Application Note

SC4525E/F Application Note
Converting SC4525C/D to SC4525E/F
AN-PM-1302
POWER MANAGEMENT
This application note services two purposes. First, it is intended to assist designers in the transition from Semtech’s SC4525C
and SC4525D step-down switching regulators to the SC4525E and SC4525F devices. Second, it provides a new external
synchronization schematic for SC4525 product family. The SC4525E and SC4525F regulators share the identical footprint as
the “C” and “D” versions, so existing customers will not need to make any PCB changes. There are no external component
changes necessary to ensure the SC4525E and SC4525F perform identically to the SC4525C and SC4525D.
V IN
D1
10V – 28V
C4
4 .7 µF
1N4148
C1
0 .3 3 µF
L1
BST
IN
SW
S C 4525C /E
S S /E N
5 .2 µH
OUT
R4
3 3 .2 k
5 V /3 A
FB
COMP
C7
10nF
C8
10pF
ROSC
GND
D2
20BQ 030
R5
1 6 .5 k
R7
1 7 .4 k
R6
8 .2 5 k
C2
1 0 µF X 3
C5
1 .5 n F
L 1 : C o iltro n ics C D 1 -5 R 2
C 2 : M u ra ta G R M 3 1 C R 6 0 J1 0 6 K
C 4 : M u ra ta G R M 3 2 E R 7 1 H 4 7 5 K
Figure 1 — SC4525C/E Typical Application
Circuit Schematic
V IN
D1
10 V – 16 V
C4
10µF
1N 4148
C1
0.33µF
L1
BST
IN
SW
S C 4525D /F
S S /E N
10µH
OUT
R4
33 .2 k
5 V /3 A
FB
COMP
C7
22 nF
C8
47pF
RSET
R7
11 .5k
GND
R5
60 .4k
D2
20 B Q 030
R6
8.25k
C2
47µF
C5
2 .2 nF
L1: C oiltronics C D 1- 100
C 2: M urata G R M 31 C R 60J476 M
C 4 : M urata G R M 31 C R 61 E 106 K
Figure 2 — SC4525D/F Typical Application
Circuit Schematic
Rev. 2.0
© 2013 Semtech Corporation
SC4525E/F Application Note
Bootstraping the power transistor
For VOUT > 8V applications using the SC4525C/D a diode, D4 as shown in Figure 3, is required in parallel with the bootstrap
capacitor. There is an improvement in the SC4525E/F circuits. There is no need of that extra diode for Vout >8V applications. This is shown in Figure 4.
D1
BST
V IN
D4
D1
BST
C1
V O U T >8V
SW
IN
SC4525C/D
GND
V IN
C1
V O U T >8V
SW
IN
SC4525E/F
D2
D2
GND
Figure 3 — SC4525C/D VOUT > 8V Application Circuit
Figure 4 — SC4525E/F VOUT > 8V Application Circuit
Electrical Characteristics Changes
The specification differences between the SC4525C/D and SC4525E/F are minor and have minimal or no impact on the
circuit performance. The comparison results are shown in Tables 4 and 5.
Table 4 — SC4525C and SC4525E Electrical Characteristic Comparison
SC4525C
Parameter
Minimum
Minimum Switch On-time (ns)
Typical
SC4525E
Maximum
Minimum
150
Typical
Maximum
150
250
Table 5 — SC4525D and SC4525F Electrical Characteristic Comparison
SC4525D
Parameter
Minimum
Minimum Switch On-time (ns)
AN-PM-1302
Typical
150
SC4525F
Maximum
Minimum
Typical
Maximum
150
250
SC4525E/F Application Note
POWER MANAGEMENT
Converting SC4525C/D to SC4525E/F
AN-PM-1302
External Synchronization
D1
V IN
There is no external clock buffer inside the SC4525E. However, the SC4525E may be synchronized to an external
clock and synchronization actually works well for regulators running below 35% duty cycle.
IN
To allow for free-running frequency tolerances, the nominal free-running frequency should be set (using Table 1)
to the synchronizing frequency:
I)5((581 | I6<1& Synchronization Test
1. Set the SC4525E free-running frequency equal to the
clock frequency fSYNC.
2. Start with a single coupling capacitor [Figure 5(a)],
test for synchronization with a 10pF coupling capacitor.
If the clock cannot lock onto the internal oscillator, then
increase CC as needed. If the clock duty cycle can be adjusted, then set it in middle of its locking range.
3. Use the circuit in Figure 5(b) if a single coupling capacitor cannot result in synchronization or there are excessive
PWM jitters.
4. Verify synchronization with fFREE-RUN programmed to
0.8fSYNC and 1.2fSYNC. Adjust clock duty cycle if necessary.
This ensures frequency locking under the worst-case freerunning frequency tolerances.
VOUT
SW
SS/EN
SC4525E/F
FB
To synchronize the SC4525E, the clock is fed into the
ROSC pin through a coupling capacitor CC [Figures 5(a)
and 5(b)]. The falling edge of the clock injects a current
pulse into the ROSC pin. This current pulse momentarily
increases the internal oscillator capacitor charging current and trips the oscillator comparator, thus locking the
clock onto the internal oscillator. CC is typically between
4.7pF and 47pF. The clock logic level can be up to 5V. The
magnitude of the injected current pulse is proportional
to the transition rate of the clock falling edge. CC can be
10pF if the clock swings from 0 to 5V with fall time less
than 20ns. The ability to synchronize may depend on the
clock duty cycle. The small Schottky diode BAT54 in Figure
5(b) reduces coupling from the clock rising edge to the
ROSC pin. It not only extends the useful duty cycle range
of the synchronizing clock but also reduces PWM (SW falling edge) jitters.
C1
BST
CC
D2
ROSC COMP GND
External C lock
R3
(a)
D1
V IN
IN
C1
BST
VOUT
SW
SS/EN
SC4525E/F
FB
BAT 54
CC
D2
ROSC COMP GND
R3
External C lock
(b)
Figure 5. Methods of Synchronizing the SC4525E.
(a) A Single Ceramic Coupling Capacitor
(b) A Coupling Capacitor in Series with a Schottky Diode.
Table 1: Resistor for Typical Switching Frequency
Freq. (k)
ROSC (k)
Freq. (k)
ROSC (k)
Freq. (k)
ROSC (k)
200
110
700
25.5
1400
9.76
250
84.5
800
21.5
1500
8.87
300
69.8
900
18.2
1600
8.06
350
57.6
1000
15.8
1700
7.15
400
49.9
1100
14.0
1800
6.34
500
38.3
1200
12.4
1900
5.62
600
30.9
1300
11.0
2000
5.23
© 2013 Semtech Corporation
SC4525E/F Application Note
Converting SC4525C/D to SC4525E/F
AN-PM-1302
POWER MANAGEMENT
Frequently Asked Questions
Q1. Does my oscillator resistor value need to change?
A. No.
The switching frequency of SC4525E/F is set with an external resistor from the ROSC pin to ground. The oscillator frequency setting resistor values of the SC4525C/D and SC4525E/F are same. When converting from the SC4525C/D to
SC4525E/F, no change of the oscillator resistor is needed.
Q2. Do I need to change my compensation network component values?
A. No.
The switching regulators in the SC4525C/D and SC4525E/F require a simple Type-2 compensation network for stable
operation. The correct calculation of these component values (R7, C5 and C8) is very important to maintain the stability
of the circuit. It is essential to verify loop compensation by checking regulator load transient response. With the largest
load step pertinent to the application applied, the regulator output voltage and the load current were observed. These
transient waveforms should not show any ringing or excessive overshoot. It is required to adjust the component values
until we get a stable operation. While verifying the load transient response of SC4525E/F, no excessive ringing or overshoot was noticed. So no component change is needed while converting SC4525C/D to SC4525E/F.
Q3. If I change SC4525C/D with SC4525E/F, will it impact the efficiency?
A. Efficiency is more or less same with both parts.
Q4. Do I need to change feedback resistor?
A. No, same feedback resistor can be used for both the parts.
Q5. Do I need to change feedback resistor?
A. No, same feedback resistor can be used for both the parts.
© 2013 Semtech Corporation
SC4525E/F Application Note
© Semtech 2013
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Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
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