LINEAR TECHNOLOGY LTC3751 Technical data

DESIGN FEATURES L
GND
0
V
OUT
100V/DIV
I
IN(AVG)
2A/DIV
20ms/DIV
VIN = 24V C
OUT
= 100µF
2V/DIV
250ns/DIV
GND
CHARGE CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RDCM
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1*
1:10
D1
V
OUT
50V TO 450V
V
TRANS
10V TO 24V
V
CC
TO µP
V
CC
LT3751
GND RBG
R6
40.2k
OFF ON
C3 680µF
C2
2.2µF s5
C1 10µF
R7
18.2k
R8
40.2k
M1
R5
6mΩ 1W
D2
+
+
C4
100µF
R9
V
TRANS
R1, 154k
R2, 475k
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED
D1,D2: VISHAY MURS260 M1: IRF3710Z T1: WURTH 750310349
LIMIT OUTPUT POWER TO 40W FOR 65°C T1 MAX AMBIENT OPERATION
*
4.7nF
Y RATED
DC/DC Converter, Capacitor Charger Takes Inputs from 4.75V to 400V
Introduction
High voltage power supplies and ca­pacitor chargers are readily found in a number of applications, including professional photoflashes, security control systems, pulsed radar systems, satellite communication systems, and explosive detonators. The LT3751 makes it possible for a designer to meet the demanding requirements of these applications, including high reliability, relatively low cost, safe operation, minimal board space and high performance.
The LT3751 is a general purpose flyback controller that can be used as either a voltage regulator or as a capac­itor charger. The LT3751 operates in boundary-mode, between continuous conduction mode and discontinuous conduction mode. Boundary-mode operation allows for a relatively small transformer and an overall reduced PCB footprint. Boundary-mode also reduces large signal stability issues that could arise from using voltage­mode or PWM techniques. Regulation is achieved with a new dual, overlap­ping modulation technique using both
by Robert Milliken and Peter Liu
Figure 1. Gate driver waveform in a typical application
peak primary current modulation and duty-cycle modulation, drastically re­ducing audible transformer noise.
The LT3751 features many safety and reliability functions, including two sets of undervoltage lockouts (UVLO), two sets of overvoltage lockouts (OVLO), no-load operation, over-temperature lockout (OTLO), in­ternal Zener clamps on all high voltage pins, and a selectable 5.6V or 10.5V internal gate driver voltage clamp (no external components needed). The LT3751 also adds a start-up/short­circuit protection circuit to protect against transformer or external FET
damage. When used as a regulator, the LT3751’s feedback loop is internally compensated to ensure stability. The LT3751 is available in two packages, either a 20-pin exposed pad QFN or a 20-lead exposed pad TSSOP.
New Gate Driver with Internal Clamp Requires No External Components
There are four main concerns when using a gate driver: output current drive capability, peak output voltage, power consumption and propagation delay. The LT3751 is equipped with a
1.5A push-pull main driver, enough to drive +80nC gates. An auxiliary 0.5A PMOS pull-up only driver is also inte­grated into the LT3751 and is used in parallel with the main driver for VCC voltages of 8V and below. This PMOS driver allows for rail-to-rail operation. Above 8V, the PMOS driver must be deactivated by tying its drain to VCC.
Most discrete FETs have a VGS limit of 20V. Driving the FET higher than 20V could cause a short in the inter­nal gate oxide, causing permanent
Figure 2. Isolated high voltage capacitor charger from 10V to 24V input
Linear Technology Magazine • March 2009
Figure 3. Isolated high voltage capacitor
charger charging waveform
9
L DESIGN FEATURES
R
N
V V
R
OUT TRIP DIODE
9 8
0 98=•
+
.
( )
0
GND
V
DRAIN
20V/DIV
I
PRIMARY
5A/DIV
10µs/DIV
V
OUT
(V)
EFFICIENCY (%)
LOAD CURRENT (mA)
1000
90
60
20 40 60 80
65
70
80
75
85
402
399
400
401
LOAD REGULATION
EFFICIENCY
0
GND
V
DRAIN
20V/DIV
I
PRIMARY
5A/DIV
10µs/DIV
CHARGE CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RDCM
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1** 1:10
D1
V
OUT
400V
V
TRANS
10V TO 24V
V
CC
TO µP
V
CC
LT3751
GND RBG
R6
40.2k
OFF ON
C3 680µF
R10* 499k
R11
1.54k
C2
2.2µF s5
C1 10µF
R7
18.2k
R8
40.2k
M1
R5 6mΩ 1W
D2
+
+
C4
100µF
R9 787Ω
V
TRANS
R1,154k
R2, 475k
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
C6 10nF
ALL RESISTORS ARE 0805, 1% RESISTORS UNLESS OTHERWISE NOTED
C4: CDE 380LX101M500J042 C5: TDK CKG57NX7R2J474M D1,D2: VISHAY MURS260 M1: IRF3710Z T1: WURTH 750310349
USE TWO SERIES 1206, 1% RESISTORS FOR R10 R10: 249k s2
LIMIT OUTPUT POWER TO 40W FOR 65°C T1 MAX AMBIENT OPERATION
*
**
Figure 4. A 10V to 24V input, 400V regulated power supply
damage. To alleviate this issue, the LT3751 has an internal, selectable
5.6V or 10.5V gate driver clamp. No external components are needed, not even a capacitor. Simply tie the CLAMP pin to ground for 10.5V operation or tie to VCC for 5.6V operation. Figure 1 shows the gate driver clamping at
10.5V with a VCC voltage of 24V. Not only does the internal clamp
protect the FET from damage, it also reduces the amount of energy injected into the gate. This increases overall efficiency and reduces power con­sumption in the gate driver circuit. The gate driver overshoot is very minimal, as seen in Figure 1. Placing the external FET closer to the LT3751 HVGATE pin reduces overshoot.
a. Switching waveform for I
10
High Voltage, Isolated Capacitor Charger from 10V to 24V Input
The LT3751 can be configured as a fully isolated stand-alone capaci­tor charger using a new differential discont inuous-c on duction- mode (DCM) comparator—used to sense the boundary-mode condition—and a new differential output voltage (V
) comparator. The differential
OUT
operation of the DCM comparator and V
comparator allow the LT3751 to
OUT
accurately operate from high voltage input supplies of greater than 400V. Likewise, the LT3751’s DCM compara­tor and V input supplies down to 4.75V. This accommodates an unmatched range of power sources.
= 100mA b. Switching waveform for I
OUT
Figure 5. High voltage regulator performance
comparator can work with
OUT
Figure 2 shows a high voltage ca­pacitor charger driven from an input supply ranging from 10V to 24V. Only five resistors are needed to operate the LT3751 as a capacitor charger. The output voltage trip point can be continuously adjusted from 50V to 450V by adjusting R9 given by:
The LT3751 stops charging the output capacitor once the programmed output voltage trip point (V reached. The charge cycle is repeated by toggling the CHARGE pin. The maximum charge/discharge rate in
= 10mA c. Efficiency and load regulation
OUT
Linear Technology Magazine • March 2009
OUT(TRIP)
) is
DESIGN FEATURES L
P
C FREQUENCY
V V V
AVG
OUT
OUT TRIP RIPPLE
=
1
2
2
( )
RRIPPLE
W240
(
)
V
CC
R3, 154k
R4, 475k
CHARGE CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1*** 1:3
D1
V
OUT
500V
V
TRANS
100V TO 400V DC
V
CC
10V TO 24V
TO µP
V
CC
LT3751
GND RBG
R6* 625k
OFF ON
C3
100µF
450V
C2
2.2µF 630V s5
C1 10µF
R8
137k ×3
R7
88.7k + 7.5k
R10*
208k
R13,20Ω
M1 FQB4N80
R12 68mΩ 1/4W
D2
+
+
C4
220µF
550V
R5
1.11k
V
TRANS
R1**
1.5M
R2**, 9M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF 630V
RDCM
F1, 1A
R9
66.5k
R11
14.7k +
17.4k
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS OTHERWISE NOTED
C4: HITACHI PS22L221MSBPF
C5: TDK CKG57NX7R2J474M T1: COILCRAFT HA4060-AL D1,D2: VISHAY US1M F1: BUSSMANN PCB-1-R
* USE THREE SERIES 1206, 0.1%
RESISTORS FOR R6 & R10 R6: 249k ×2 + 127k R10: 66.5k ×2 + 75k
** USE TWO SERIES 1206, 1%
RESISTORS FOR R1 & R2 R1: 750k ×2 R2: 4.53M ×2
*** OUTPUT POWER LIMITED TO
20W FOR 65°C T1 AMBIENT OPERATION
4.7nF
Y RATED
0
V
OUT
AC RIPPLE
10V/DIV
I
IN(AVG)
20mA/DIV
2s/DIV
CHARGE TIME (ms)
V
OUT,TRIP
(V)
INPUT VOLTAGE (V)
400100
530
490
200 300
500
520
510
1000
400
850
550
700
V
OUT,TRIP
CHARGE TIME
the output capacitor is limited by the temperature rise in the transformer. Limiting the transformer surface tem­perature in Figure 2 to 65°C with no air flow requires the average output power to be 40W given by:
where V voltage, V
OUT(TRIP)
is the output trip
is the ripple voltage
RIPPLE
on the output node, and frequency is the charge/discharge frequency. Two techniques are used to increase the available output power: increase the airflow across the transformer, or in­crease the size of the transformer itself. Figure 3 shows the charging waveform and average input current for a 100µF output capacitor charged to 400V in less than 100ms (R9 = 976).
For output voltages higher than 450V, the transformer in Figure 2 must be replaced with one having higher primary inductance and a higher turns ratio. Consult the LT3751 data
Figure 6. The LT3751 protecting the output during a no-load condition
sheet for proper transformer design procedures.
High Voltage Regulated Power Supply from 10V to 24V Input
The LT3751 can also be used to convert a low voltage supply to a much higher voltage. Placing a resistor divider from the output node to the FB pin and ground causes the LT3751 to oper­ate as a voltage regulator. Figure 4 shows a 400V regulated power supply operating from an input supply range of 10V to 24V.
The LT3751 uses a regulation con­trol scheme that drastically reduces audible noise in the transformer and the input and output ceramic bulk
capacitors. This is achieved by using an internal 26kHz clock to synchronize the primary winding switch cycles. Within the clock period, the LT3751 modulates both the peak primary current and the number of switch­ing cycles. Figures 5a and 5b show heavy-load and light-load waveforms, respectively, while Figure 5c shows efficiency over most of the operating range for the application in Figure 4.
The clock forces at least one switch cycle every period which would over­charge the output capacitor during a no-load condition. The LT3751 han­dles no-load conditions and protects against over-charging the output node. Figure 6 shows the LT3751 protecting during a no-load condition.
Resistors can be added to RV
OUT
and RBG to add a second layer of protec­tion, or they can be omitted to reduce component count by tying RV
OUT
and RBG to ground. The trip level for the V
comparator is typically set 20%
OUT
higher than the nominal regulation voltage. If the resistor divider were to fail, the V
comparator would disable
OUT
switching when the output climbed to 20% above nominal.
Linear Technology Magazine • March 2009
Figure 7. A 100V to 400V input, 500V output, isolated capacitor charger
Figure 8. Isolated capacitor charger V and charge time with respect to input voltage
OUT(TRIP)
11
L DESIGN FEATURES
OUTPUT VOLTAGE (V)
INPUT VOLTAGE (V)
400100
398
395
200 300
396
397
I
OUT
= 10mA
I
OUT
= 25mA
I
OUT
= 50mA
EFFICIENCY (%)
OUTPUT CURRENT (mA)
750
90
40
50
25 50
60
70
80
VIN = 100V VIN = 250V VIN = 400V
V
CC
R3, 154k
R4, 475k
CHARGE CLAMP
V
CC
DONE
FAULT
UVLO1
OVLO1
UVLO2
OVLO2
RV
OUT
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
T1*** 1:3
D1
V
OUT
400V
V
TRANS
100V TO 400V DC
V
CC
10V TO 24V
TO µP
V
CC
LT3751
GND RBG
R6* 615k
OFF ON
C3 100µF
C2
2.2µF s5
C1 10µF
C6 10nF
R8*
411k
R13,20Ω
M1 FQB4N80
R10 68mΩ ¼W
D2
+
+
C4
100µF
R12
1.54k
R11** 499k
V
TRANS
R1**, 1.5M
R2**, 9M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
C5
0.47µF
RDCM
F1, 1A
R9
66.5k
ALL RESISTORS ARE 0805,
1% RESISTORS UNLESS OTHERWISE NOTED
C4: CDE 380LX101M500J042
C5: TDK CKG57NX7R2J474M T1: COILCRAFT HA4060-AL D1,D2: VISHAY US1M F1: BUSSMANN PCB-1-R
* USE THREE SERIES 1206, 1%
RESISTORS FOR R6 & R8 R6: 205k ×3
R8: 137k ×3
** USE TWO SERIES 1206, 1%
RESISTORS FOR R1, R2 & R11 R1: 750k ×2 R2: 4.53M ×2 R11: 249k ×2
*** OUTPUT POWER LIMITED TO
20W FOR 65°C T1 AMBIENT OPERATION
R7
95.3k
can also be used for a capacitor charger. The LT3751 operates as a capacitor charger until the FB pin reaches 1.225V, after which the LT3751 operates as a voltage regulator. This keeps the capacitor topped-off until the application needs to use its energy. The output resistor divider forms a leakage path from the output capacitor to ground. When the output voltage droops, the LT3751 feedback circuit will keep the capacitor topped-
12
Figure 9. A 100V to 400V input, 400V output, capacitor charger and voltage regulator
Note that the FB pin of the LT3751
a. Overall efficiency b. Line regulation
Figure 10. High voltage input and output regulator performance
off with small, low current bursts of charge as shown in Figure 6.
High Input Supply Voltage, Isolated Capacitor Charger
As mentioned above, the LT3751 dif­ferential DCM and V allow the part to accurately work from very high input supply voltages. An offline capacitor charger, shown in Figure 7, can operate with DC input voltages from 100V to 400V. The trans­former provides galvanic isolation from
comparators
OUT
the input supply to output node—no additional magnetics required.
Input voltages greater than 80V require the use of resistor dividers on the DCM and V
comparators
OUT
(charger mode only). The accuracy of the V
trip threshold is heightened
OUT
by increasing current IQ through R10 and R11; however, the ratio of R6/R7 should closely match R10/R11 with tolerances approaching 0.1%. A trick is to use resistor arrays to yield the desired ratio. Achieving 0.1% ratio ac­curacy is not difficult and can reduce the overall cost compared to using individual 0.1% surface mount resis­tors. Note that the absolute value of the individual resistors is not critical, only the ratio of R6/R7 and R10/R11. The DCM comparator is less critical and can tolerate resistance variations greater than 1%.
The 100V to 400VDC input capaci­tor charger has an overall V accuracy of better than 6% over the entire operating range using 0.1% re­sistor dividers. Figure 8 shows a typical performance for V
OUT(TRIP)
and charge
time for the circuit in Figure 7.
Linear Technology Magazine • March 2009
OUT(TRIP)
DESIGN FEATURES L
V
TRANS
100V TO 200V DC
V
CC
V
CC
R11, 84.5k
R12, 442k
UVLO1
OVLO1
UVLO2
OVLO2
DONE
FAULT
CHARGE CLAMP
V
CC
HVGATE
LVGATE
CSP
CSN
FB
RV
TRANS
TO µP
V
CC
LT3751
LT4430
GND RBG
R3 210k
OFF ON
C3 22µF 350V s2
C2 1µF
C1 100pF
C4 1µF 250V s2
C7 400µF 330V
C8 22nF
R17
3.16k
R14 249k
V
OUT
282V 225mA
C6
0.1µF 630V
ISOLATION BOUNDARY
C5
0.01µF 630V
C9
3.3µF
C10
0.47µF
M2
M1
D1
R8
2.49k
R7 475Ω
R18
274Ω
R6 40mΩ 1/4W
V
TRANS
R9, 2.7M
R5, 210k
R13
5.11Ω
R16, 1k
R10, 4.3M
DANGER HIGH VOLTAGE! OPERATION BY HIGH VOLTAGE TRAINED PERSONNEL ONLY
RDCM
RV
OUT
U1
ALL RESISTORS ARE 0805,1% RESISTORS UNLESS OTHERWISE NOTED
C7: 330FK400M22X38 D1: 12V ZENER D2: MURS140 D3: P6kE200A D4, D5: STTH112A D6: BAT54 D7: BAS516 M1: IRF830 M2: STB11NM60FD T1: TDK SRW24LQ-UxxH015 (Np:Ns:Npb:Nsb=1:2:0.08:0.08) U1: PS2801-1 U2: LT4430
R4, 105k
R2, 10ΩD2
D3
D5
D6
D4
Np
Ns
Nsb
U2
VINCOMP
GND
OC FB
OPTO
Npb
T1
D7
R15 221k
R1
49.9k 1/2W
+
+
1
1 1 1
1
11
1
1
1
2
2
2
2
2
2
2
F1, 2A
1 2
4.7nF
Y RATED
0
GND
V
DRAIN
100V/DIV
I
PRIMARY
2A/DIV
20µs/DIV
0
GND
V
DRAIN
100V/DIV
I
PRIMARY
2A/DIV
20µs/DIV
R
V
A
OUT TRIP
11
1 225
50
=
( )
.
µ
Figure 11. Fully isolated, high output voltage regulator
High Input Supply Voltage, Non-Isolated Capacitor Charger/Regulator
The FB pin of the LT3751 can also be configured for charging a capaci­tor from a high input supply voltage. Simply tie a resistor divider from the output node to the FB pin. The resis­tor dividers on the R pins can tolerate 5% resistors, and all the R removed. This lowers the number and
and RBG pin resistors are
V(OUT)
the tolerance of required components, reducing board real estate and overall design costs. With the output voltage resistor divider, the circuit in Figure 9 is also a fully functional, high-ef ficiency voltage regulator with load
Linear Technology Magazine • March 2009
VTRANS
a. I
and R
= 225mA b. I
OUT
DCM
-
and line regulation better than 1%. Efficiency and line regulation for the circuit in Figure 9 are shown in Figure 10a and Figure 10b, respectively.
Alternatively, a resistor can be tied from V pin. This mimics the V
to the OVLO1 pin or OVLO2
OUT
compara-
OUT
tor, stopping charging once the target voltage is reached. The FB pin is tied to ground. The CHARGE pin must be toggled to initiate another charge se­quence, thus the LT3751 operates as a capacitor charger only. Resistor R12 is omitted from Figure 9 and resistor R11 is tied from V or OVLO2. R11 is calculated using the following equation:
Figure 12. Switching waveforms
OUT
directly to OVLO1
Note that OVLO1 or OVLO2 will cause the FAULT pin to indicate a fault when the target outpaut voltage, V
OUT(TRIP) ,
is reached.
High Voltage Input/Output Regulator with Isolation
Using a resistor divider from the output node to the FB pin allows regulation but does not provide galvanic isolation. Two auxiliary windings are added to the transformer in circuit shown in Figure 11 to drive the FB pin, the
OUT
= 7.1mA
continued on page 42
13
L NEW DEVICE CAMEOS
EFFICIENCY (%)
INPUT DC VOLTAGE (V)
200100
100
70
120 140 160 180
75
80
90
85
95
P
OUT
= 63W
P
OUT
= 48W
P
OUT
= 25W
OUTPUT VOLTAGE ERROR (V)
I
OUT
(mA)
2500
–0.5
–0.25
0
0.25
0.5
10050 150 200
battery whether external or internal. Programming the charge current only requires a single external resistor.
The fault management system of the LTC4012 family suspends charging immediately for various conditions. First is battery overvoltage protection, which can occur with the sudden loss of battery load during bulk charge. Second, each IC features internal over-temperature protection to pre­vent silicon damage during elevated thermal operation.
The LTC4012 family has a logic-level shutdown control input and three open-drain status outputs. First is an input current limit (ICL) status flag to tell the system when VIN is running at over 95% of its current capacity. The input current limit accuracy is typi­cally ±3% and a maximum of ±4% over the full operating temperature range. Next is the AC present status, which indicates when VIN is within a valid range for charging under all modes of operation. The last is a charge status output can indicate bulk or C/10 charge states. The control input and status outputs of the LTC4012, along
with the analog current monitor out­put, can be used by the host system to perform necessary preconditioning, charge termination and safety timing functions.
4MHz Synchronous Step­Down DC/DC Converter Delivers up to 1.25A from a 3mm × 3mm DFN
The LTC3565 is a high efficiency syn­chronous step-down regulator that can deliver up to 1.25A of continuous output current from a 3mm × 3mm DFN (or MSOP-10E) package. Using a constant frequency of (up to 4MHz) and current mode architecture, the LTC3565 operates from an input volt­age range of 2.5V to 5.5V making it ideal for single cell Li-Ion, or multicell Alkaline/NiCad/NiMH applications. It can generate output voltages as low as 0.6V, enabling it to power the latest generation of low voltage DSPs and microcontrollers. An independent RUN pin enables simple turn-on and shutdown. Its switching frequency is user programmable from 400kHz to 4MHz, enabling the designer to
optimize efficiency while avoiding criti­cal noise-sensitive frequency bands. The combination of its 3mm × 3mm DFN-10 (or MSOP-10) package and high switching frequency keeps ex­ternal inductors and capacitors small, providing a very compact, thermally efficient footprint.
The LTC3565 uses internal switches
with an R
of only 0.13 (N-Chan-
DS(ON)
nel lower FET) and 0.15 (P-Channel upper FET) to deliver efficiencies as high as 95%. It also utilizes low dropout 100% duty cycle operation to allow output voltages equal to VIN, further extending battery run time. The LTC3565 utilizes Automatic Low Ripple ( < 25mV
) Burst Mode®
P–P
operation to offer only 40µA no load quiescent current. If the application is noise sensitive, Burst Mode operation can be disabled using a lower noise pulse-skipping mode, which still offers only 330µA of quiescent current. The LTC3565 can be synchronized to an external clock throughout its entire frequency range. Other features in­clude ±2% output voltage accuracy and
over-temperature protection.
L
LT3751, continued from page 13
LT3751 controller, and the optocoupler on the feedback resistor divider. The auxiliary windings provide the desired galvanic isolation boundary while maintaining an isolated feedback path from the output node to the LT3751 FB pin. Figures 12 and 13 show the regulator’s performance.
The fully isolated, high voltage in­put/output regulator yields over 90% efficiency. Load regulation is excellent as shown in Figure 13b, due mainly to the added gain of the optocoupler circuit.
Conclusion
The ability to run from any input supply voltage ranging from 4.75V to greater than 400V and the abun­dance of safety features make the LT3751 an excellent choice for high voltage capacitor chargers or high voltage regulated power supplies. In fact, the LT3751 is, for now, the only
42
42
a. Efficiency b. Load regulation
Figure 13. Fully isolated, high voltage regulator performance
boundary-mode capacitor charger controller that can accurately operate from extremely high input voltages. The LT3751 simplifies design by in­tegrating many functions that—due to cost and board real-estate—would otherwise not be realizable. Although
LT3751 includes many more features than we can show in one article. We recommended consulting the data sheet or calling the Linear Technology applications engineering department for more in-depth coverage of all avail­able features.
L
several designs are shown here, the
Linear Technology Magazine • March 2009
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