LTC3870_15 Datasheet, PDF (12/22 Pages) Linear Technology – PolyPhase Step-Down Slave Controller for LTC3880/LTC3883 with Digital Power System Management

LTC3870
Applications Information
The Typical Application on the first page of this data sheet is
a basic LTC3870 application circuit featuring the LTC3880
as a slave controller. In paralleled operation, the current
sensing scheme as well as the power stage parameters
in LTC3870 must be the same as the master controller to
achieve balanced current sharing between masters and
slaves. Finally, input and output capacitors are selected
based on RMS current rating, ripple, and transient specs.
Current Limit Programming
To match the master controller current limit, each chan-
nel of LTC3870 can be programmed separately with two
current ranges. The I_LIM pin of LTC3870 is a 4-level logic
input which sets the current limit of LTC3870. When I_LIM
is grounded, both channel0 and channel1 are set to be low
current range. When I_LIM is tied to INTV_CC, both channel0
and channel1 are set to be high current range. Here, low
current range means the current sense threshold linearly
increases from 0mV to 50mV as I_TH voltage is increased
from 0.5V to 2.22V without slope compensation. High cur-
rent range means the current sense threshold increases to
75mV as I_TH voltage is increased to 2.22V without slope
compensation. Set I_LIM to one-third INTV_CC for channel0
high current range and channel1 low current range. Set
I_LIM to two-thirds INTV_CC or float for channel0 low current
range and channel1 high current range. The summary of
I_LIM pin setups is shown in Table 2. For balanced load
current sharing, use the same current range setting as
in the master controller. Note that the LTC3870 does not
have active clamping circuit on I_TH pin for peak current
limit and over current protection. Over current protection
relies on the master controller to drive and clamp the I_TH
pin voltage not to exceed the programmed voltage through
the PMBus command.
Table 2.
I_LIM
GND
1/3 INTV_CC
2/3 INTV_CC or Float
INTV_CC
Channel 0
Current limit
Range Low
Range High
Range Low
Range High
Channel 1
Current limit
Range Low
Range Low
Range High
Range High
INTV_CC Regulators and EXTV_CC
The LTC3870 features a PMOS LDO that supplies power
to INTV_CC from the V_IN supply. INTV_CC powers the gate
drivers and most of the LTC3870’s internal circuitry. The
linear regulator regulates the voltage at the INTV_CC pin
to 5.0V when V_IN is greater than 6V. EXTV_CC connects to
INTV_CC through another PMOS LDO and can supply the
needed power when its voltage is higher than 4.8V and
V_IN is higher than 6.5V. Each of these LDOs can supply a
peak current of 100mA and must be bypassed to ground
with a minimum of 4.7µF ceramic capacitor or low ESR
electrolytic capacitor. No matter what type of bulk capaci-
tor is used, an additional 0.1µF ceramic capacitor placed
directly adjacent to the INTV_CC and PGND 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 LTC3870 to be exceeded.
The INTV_CC current, which is dominated by the gate charge
current, may be supplied by either the 5.0V linear regula-
tor from V_IN or the linear regulator from EXTV_CC. When
the voltage on the EXTV_CC pin is less than 4.8V, the linear
regulator from V_IN is enabled. Power dissipation 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.
The junction temperature can be estimated by using the
equations given in Note 2 of the Electrical Characteristics.
For example, the LTC3870 INTV_CC current is limited to less
than 34mA from a 38V supply in the UFD package and not
using the EXTV_CC supply:
T_J = 70°C + (34mA)(38V)(43°C/W) = 125°C
where ambient temperature is 70°C and thermal resistance
from junction to ambient is 43°C/W.
To prevent the maximum junction temperature from be-
ing exceeded, the input supply current must be checked
while operating in continuous conduction mode (MODE
= INTV_CC) at maximum V_IN. When the voltage applied to
EXTV_CC rises above 4.8V and V_IN above 6.5V, the INTV_CC
linear regulator is turned off and the EXTV_CC linear regulator
is turned on. Using the EXTV_CC allows the MOSFET driver
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