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| 型号： | MAX17499BEUB+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 12/18页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST ISO PWM 10-UMAX |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 625kHz |
| 占空比： | 75% |
| 电源电压： | 9.5 V ~ 24 V |
| 降压： | 无 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 10-TFSOP，10-MSOP（0.118"，3.00mm 宽） |
| 包装： | 带卷 (TR) |
Current-Mode PWM Controllers with
Programmable Switching Frequency
circuit in Figure 6, calculate the output voltage using
the following equation:
the maximum startup bias of the device (90μA) and the
charging current for C1 and C2. The bypass capacitor,
? ? 1 +
? ? V REF D 6 ? ? V D 2
+ V
?
V OUT =
N S
N T
? ?
? ?
R 2
R 1 ? ?
C2, must charge to 9.5V and C1 to 24V, all within the
desired time period of 500ms. Because of the internal
soft-start time of the MAX17500 (approximately 5.6ms
when f SW = 350kHz), C1 must store enough charge to
where N S is the number of secondary winding turns, N T
is the number of tertiary winding turns, and both V D6
and V D2 are the diode drops at the respective outputs.
Current Limit
deliver current to the device for at least this much time.
To calculate the approximate amount of capacitance
required, use the following formula:
I G = Q GTOT f SW
C 1 = IN G SS
The current-sense resistor (R4 in Figure 1), connected
between the source of the MOSFET and ground, sets the
current limit. The current-limit comparator has a voltage
( I + I )( t
V HYST
)
R 4 = CS
trip level (V CS ) of 1V. Use the following equation to cal-
culate the value of R4:
V
I PRI
where I PRI is the peak current in the primary side of the
transformer, which also flows through the MOSFET.
where I IN is the MAX17500’s internal supply current
(2mA) after startup, Q GTOT is the total gate charge for
Q1, f SW is the MAX17500 ’s switching frequency
(350kHz), V HYST is the bootstrap UVLO hysteresis
(approximately 12V), and t SS is the internal soft-start
time (5.6ms).
Example: I G = (8nC) (350kHz) ? 2.8mA
When the voltage produced by this current (through the
current-sense resistor) exceeds the current-limit com-
parator threshold, the MOSFET driver (NDRV) termi-
C 1 =
(2mA + 2.8mA)(5.6ms)
12 V
= 2 . 24 μF
nates the current on-cycle within 60ns (typ). Use a
I C 1 = SUVR
small RC network to filter out the leading-edge spikes
on the sensed waveform when needed. Set the corner
frequency between 2MHz and 10MHz.
Applications Information
Startup Time Considerations for Power
Choose a 2.2 μF standard value (assuming 350kHz
switching frequency).
Assuming C1 > C2, calculate the value of R1 as follows:
V C1
( 500 ms )
Supplies Using the MAX17500
The bypass capacitor at IN, C1, supplies current imme-
diately after the MAX17500 wakes up (see Figure 1).
The size of C1 and the connection configuration of the
R 1 ?
V IN(MIN) ? V SUVR
I C 1 + I S T ART
I C1 =
= 0 . 105 mA
= 61 . 5 k Ω
R 1 ?
tertiary winding determine the number of cycles avail-
able for startup. Large values of C1 increase the start-
up time but also supply gate charge for more cycles
during initial startup. If the value of C1 is too small, V IN
drops below 9.74V because NDRV does not have
enough time to switch and build up sufficient voltage
across the tertiary output, which powers the device.
The device goes back into UVLO and does not start.
Use a low-leakage capacitor for C1 and C2.
Typically, offline power supplies keep startup times to
less than 500ms even in low-line conditions (85V AC
input for universal offline or 36V DC for telecom appli-
cations). Size the startup resistor, R1, to supply both
where V IN(MIN) is the minimum input supply voltage for
the application (36V for telecom), V SUVR is the boot-
strap UVLO wake-up level (23.6V max), and I START is
the IN supply current at startup (90μA max).
For example:
(24 V )(2.2μF)
( 500 ms )
( 36 V ) ? ( 2 4 V )
( 0 . 105 mA ) + ( 90 μA )
Choose a 61.9k Ω standard value.
12
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