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| 型号： | LTC3834EUFD#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 17/28页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BUCK PWM CM 20-QFN |
| 标准包装： | 73 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 580kHz |
| 占空比： | 99.4% |
| 电源电压： | 4 V ~ 36 V |
| 降压： | 是 |
| 升压： | 无 |
| 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 ~ 85°C |
| 封装/外壳： | 20-WFQFN 裸露焊盘 |
| 包装： | 管件 |
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LTC3834
APPLICATIONS INFORMATION
INTV CC Regulators
The LTC3834 features two separate internal P-channel low
dropout linear regulators (LDO) that supply power at the
INTV CC pin from either the V IN supply pin or the EXTV CC
pin, respectively, depending on the connection of the
EXTV CC pin. INTV CC powers the gate drivers and much of
the LTC3834’s internal circuitry. The V IN LDO regulates
the voltage at the INTV CC pin to 5.25V and the EXTV CC
LDO regulates it to 7.5V. Each of these can supply a peak
current of 50mA and must be bypassed to ground with a
minimum of 4.7μF ceramic capacitor. The ceramic capacitor
placed directly adjacent to the INTV CC and PGND IC pins is
highly recommended. Good bypassing is needed to supply
the high transient currents required by the MOSFET gate
drivers and to prevent interaction between the channels.
High input voltage applications in which large MOSFETs are
being driven at high frequencies may cause the maximum
junction temperature rating for the LTC3834 to be exceeded.
The INTV CC current, which is dominated by the gate charge
current, may be supplied by either the 5V V IN LDO or the
7.5V EXTV CC LDO. When the voltage on the EXTV CC pin
is less than 4.7V, the V IN LDO is enabled. Power dissipa-
tion for the IC in this case is highest and is equal to V IN ?
I INTVCC . The gate charge current is dependent on operating
frequency as discussed in the Ef?ciency Considerations
section. The junction temperature can be estimated by
using the equations given in Note 3 of the Electrical Char-
acteristics. For example, the LTC3834 INTV CC current is
limited to less than 41mA from a 24V supply when in the
G package and not using the EXTV CC supply:
T J = 70°C + (41mA)(36V)(95°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV CC ) at maximum V IN .
When the voltage applied to EXTV CC rises above 4.7V, the
V IN LDO is turned off and the EXTV CC LDO is enabled. The
EXTV CC LDO remains on as long as the voltage applied to
EXTV CC remains above 4.5V. The EXTV CC LDO attempts
to regulate the INTV CC voltage to 7.5V, so while EXTV CC
is less than 7.5V, the LDO is in dropout and the INTV CC
voltage is approximately equal to EXTV CC . When EXTV CC
is greater than 7.5V up to an absolute maximum of 10V,
INTV CC is regulated to 7.5V.
Using the EXTV CC LDO allows the MOSFET driver and
control power to be derived from the LTC3834 switch-
ing regulator output (4.7V ≤ V OUT ≤ 10V) during normal
operation and from the V IN LDO when the output is out
of regulation (e.g., start-up, short circuit). If more current
is required through the EXTV CC LDO than is speci?ed,
an external Schottky diode can be added between the
EXTV CC and INTV CC pins. Do not apply more than 10V to
the EXTV CC pin and make sure that EXTV CC ≤ V IN .
Signi?cant ef?ciency and thermal gains can be realized
by powering INTV CC from the output, since the V IN cur-
rent resulting from the driver and control currents will be
scaled by a factor of (Duty Cycle)/(Switcher Ef?ciency). For
4.7V to 10V regulator outputs, this means connecting the
EXTV CC pin directly to V OUT . Tying the EXTV CC pin to a 5V
supply reduces the junction temperature in the previous
example from 125°C to:
T J = 70°C + (24mA)(5V)(95°C/W) = 81°C
However, for 3.3V and other low voltage outputs, addi-
tional circuitry is required to derive INTV CC power from
the output.
The following list summarizes the four possible connec-
tions for EXTV CC :
1. EXTV CC Left Open (or Grounded). This will cause
INTV CC to be powered from the internal 5.25V regulator
resulting in an ef?ciency penalty of up to 10% at high
input voltages.
2. EXTV CC Connected Directly to V OUT . This is the normal
connection for a 5V regulator and provides the highest
ef?ciency.
3. EXTV CC Connected to an External supply. If an external
supply is available in the 5V to 7V range, it may be used
to power EXTV CC providing it is compatible with the
MOSFET gate drive requirements.
4. EXTV CC Connected to an Output-Derived Boost Network.
For 3.3V and other low voltage regulators, ef?ciency
gains can still be realized by connecting EXTV CC to an
output-derived voltage that has been boosted to greater
than 4.7V. This can be done with the capacitive charge
pump shown in Figure 6.
3834fb
17
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