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| 型号： | LTC3876EFE#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 27/48页 |
| 文件大小： | 0K |
| 描述： | IC CTLR DC/DC DDR DUAL 38-TSSOP |
| 产品培训模块： | LTC3876 Dual DC/DC Controller |
| 标准包装： | 50 |
| 应用： | 控制器，DDR，DDR2，DDR3 |
| 输入电压： | 4.5 V ~ 38 V |
| 输出数： | 2 |
| 输出电压： | 可调 |
| 工作温度： | -40°C ~ 125°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 38-TFSOP （0.173"，4.40mm 宽）裸露焊盘 |
| 供应商设备封装： | 38-TSSOP 裸露焊盘 |
| 包装： | 管件 |
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LTC3876
APPLICATIONS INFORMATION
When the voltage applied to the EXTV CC pin rises above
4.7V, the V IN LDO is turned off and the EXTV CC is connected
to DRV CC2 pin with an internal switch. This switch remains
on as long as the voltage applied to EXTV CC remains
above 4.5V. Using EXTV CC allows the MOSFET driver and
control power to be derived from the LTC3876’s switching
regulator output V OUT during normal operation and from
the LDO when the output is out of regulation (e.g., start-
up, short-circuit). If more current is required through the
EXTV CC than is specified, an external Schottky diode can
be added between the EXTV CC and DRV CC pins. Do not
apply more than 6V to the EXTV CC pin and make sure that
EXTV CC is less than V IN .
For applications where the main input power never exceeds
5.3V, tie the DRV CC1 and DRV CC2 pins to the V IN input
through a small resistor, (such as 1Ω to 2Ω) as shown
in Figure 7 to minimize the voltage drop caused by the
gate charge current. This will override the LDO and will
prevent DRV CC from dropping too low due to the dropout
voltage. Make sure the DRV CC voltage exceeds the R DS(ON)
test voltage for the external MOSFET which is typically at
4.5V for logic-level devices.
LTC3876
DRV CC2
Significant efficiency and thermal gains can be realized
by powering DRV CC from the switching converter output,
DRV CC1
R DRVCC
C DRVCC
C IN
V IN
since the V IN current resulting from the driver and control
currents will be scaled by a factor of (duty cycle)/(switcher
efficiency).
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 + (52mA)(5V)(28°C/W) = 77°C
However, for 3.3V and other low voltage outputs, ad-
ditional circuitry is required to derive DRV CC power from
the converter 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 5.3V LDO resulting
in an efficiency penalty of up to 10% at high input
voltages.
2. EXTV CC connected directly to switching converter output
V OUT > 4.7V. This provides the highest efficiency.
3. EXTV CC connected to an external supply. If a 4.7V or
greater external supply is available, it may be used to
power EXTV CC providing that the external supply is
sufficient for MOSFET gate drive requirements.
4. EXTV CC connected to an output-derived boost network.
For 3.3V and other low voltage converters, efficiency
gains can still be realized by connecting EXTV CC to an
output-derived voltage that has been boosted to greater
3876 F07
Figure 7. Setup for V IN ≤ 5.3V
Input Undervoltage Lockout (UVLO)
The LTC3876 has two functions that help protect the con-
troller in case of input undervoltage conditions. An internal
UVLO comparator constantly monitors the INTV CC and
DRV CC voltages to ensure that adequate voltages are pres-
ent. The comparator enables internal UVLO signal, which
locks out the switching action of both channels, until the
INTV CC and DR VCC1,2 pins are all above their respective
UVLO thresholds. The rising threshold (to release UVLO)
of the INTV CC is typically 4.2V, with 0.5V falling hysteresis
(to re-enable UVLO). The UVLO thresholds for DR VCC1,2 are
lower than that of INTV CC but higher than typical threshold
voltages of power MOSFETs, to prevent them from turning
on without sufficient gate drive voltages.
Generally for V IN > 6V, a UVLO can be set through monitoring
the V IN supply by using external voltage dividers at the RUN
pins from V IN to SGND. To design the voltage divider, note
that both RUN pins have two levels of threshold voltages.
The precision gate-drive-enable threshold voltage of 1.2V
than 4.7V.
3876f
27
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