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| 型号： | LTC3859EUHF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 28/42页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM CM 38-QFN |
| 标准包装： | 52 |
| PWM 型： | 电流模式，Burst Mode? |
| 输出数： | 3 |
| 频率 - 最大： | 850kHz |
| 占空比： | 100% |
| 电源电压： | 4.5 V ~ 38 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 38-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3859
APPLICATIONS INFORMATION
I SC = 40% ?I LIM(MAX) ? Δ I L(SC)
on, the boost voltage is above the input supply: V BOOST
= V IN + V INTVCC (V BOOST = V OUT + V INTVCC for the boost
controller). The value of the boost capacitor C B needs to
be 100 times that of the total input capacitance of the
topside MOSFET(s). The reverse breakdown of the external
Schottky diode must be greater than V IN(MAX) for the buck
channels and V OUT(MAX) for the boost channel.
When adjusting the gate drive level, the ?nal arbiter is the
total input current for the regulator. If a change is made
and the input current decreases, then the ef?ciency has
improved. If there is no change in input current, then there
is no change in ef?ciency.
The topside MOSFET driver for the boost channel includes
an internal charge pump that delivers current to the boot-
strap capacitor from the BOOST3 pin. This charge current
maintains the bias voltage required to keep the top MOSFET
on continuously during dropout/overvoltage conditions.
The Schottky diode selected for the boost topside driver
should have a reverse leakage less than the available output
current the charge pump can supply under all operating
conditions. Curves displaying the available charge pump
current under different operating conditions can be found
in the Typical Performance Characteristics section.
Fault Conditions: Buck Current Limit and Current
Foldback
The LTC3859 includes current foldback for the buck
channels to help limit load current when the output is
shorted to ground. If the buck output falls below 70% of
its nominal output level, then the maximum sense volt-
age is progressively lowered from 100% to 40% of its
maximum selected value. Under short-circuit conditions
with very low duty cycles, the buck channel will begin
cycle skipping in order to limit the short-circuit current.
In this situation the bottom MOSFET will be dissipating
most of the power but less than in normal operation. The
short-circuit ripple current is determined by the minimum
on-time t ON(MIN) of the LTC3859 (≈95ns), the input voltage
and inductor value:
D I L(SC) = t ON(MIN) (V IN /L)
The resulting average short-circuit current is:
1
2
Fault Conditions: Buck Overvoltage Protection
(Crowbar)
The overvoltage crowbar is designed to blow a system
input fuse when the output voltage of the one of the buck
regulators rises much higher than nominal levels. The
crowbar causes huge currents to ?ow, that blow the fuse
to protect against a shorted top MOSFET if the short oc-
curs while the controller is operating.
A comparator monitors the buck output for overvoltage
conditions. The comparator detects faults greater than
10% above the nominal output voltage. When this condi-
tion is sensed, the top MOSFET of the buck controller is
turned off and the bottom MOSFET is turned on until the
overvoltage condition is cleared. The bottom MOSFET
remains on continuously for as long as the overvoltage
condition persists; if V OUT returns to a safe level, normal
operation automatically resumes.
A shorted top MOSFET for the buck channel will result in
a high current condition which will open the system fuse.
The switching regulator will regulate properly with a leaky
top MOSFET by altering the duty cycle to accommodate
the leakage.
Fault Conditions: Over Temperature Protection
At higher temperatures, or in cases where the internal
power dissipation causes excessive self heating on chip
(such as INTV CC short to ground), the over temperature
shutdown circuitry will shut down the LTC3859. When the
junction temperature exceeds approximately 170°C, the
over temperature circuitry disables the INTV CC LDO, caus-
ing the INTV CC supply to collapse and effectively shutting
down the entire LTC3859 chip. Once the junction tempera-
ture drops back to approximately 155°C, the INTV CC LDO
turns back on. Long term overstress (T J > 125°C) should
be avoided as it can degrade the performance or shorten
the life of the part.
Phase-Locked Loop and Frequency Synchronization
The LTC3859 has an internal phase-locked loop (PLL)
comprised of a phase frequency detector, a lowpass ?lter,
and a voltage-controlled oscillator (VCO). This allows the
turn-on of the top MOSFET of controller 1 to be locked to
the rising edge of an external clock signal applied to the
3859fa
28
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