LTC3717 Datasheet, PDF (12/20 Pages) Linear Technology – Wide Operating Range, No RSENSE Step-Down Controller for DDR/QDR Memory Termination

LTC3717
APPLICATIO S I FOR ATIO
time that the LTC3717 is capable of turning on the bottom
MOSFET, tripping the current comparator and turning the
MOSFET back off. This time is generally about 300ns. The
minimum off-time limit imposes a maximum duty cycle of
t_ON/(t_ON + t_OFF(MIN)). If the maximum duty cycle is reached,
due to a dropping input voltage for example, then the
output will drop out of regulation. The minimum input
voltage to avoid dropout is:
V_IN(MIN)
=
V_OUT
t_ON
+ t_OFF(MIN)
t_ON
INTV_CC Regulator
An internal P-channel low dropout regulator produces the
5V supply that powers the drivers and internal circuitry
within the LTC3717. The INTV_CC pin can supply up to
50mA RMS and must be bypassed to ground with a
minimum of 4.7µF tantalum or other low ESR capacitor.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers. Applica-
tions using large MOSFETs with a high input voltage and
high frequency of operation may cause the LTC3717 to
exceed its maximum junction temperature rating or RMS
current rating. Most of the supply current drives the
MOSFET gates unless an external EXTV_CC source is used.
In continuous mode operation, this current is I_GATECHG =
f(Q_g(TOP) + Q_g(BOT)). The junction temperature can be
estimated from the equations given in Note 2 of the
Electrical Characteristics. For example, the LTC3717CGN
is limited to less than 14mA from a 30V supply:
T_J = 70°C + (14mA)(30V)(130°C/W) = 125°C
For larger currents, consider using an external supply with
the EXTV_CC pin.
EXTV_CC Connection
The EXTV_CC pin can be used to provide MOSFET gate drive
and control power from the output or another external
source during normal operation. Whenever the EXTV_CC
pin is above 4.7V the internal 5V regulator is shut off and
an internal 50mA P-channel switch connects the EXTV_CC
pin to INTV_CC. INTV_CC power is supplied from EXTV_CC until
this pin drops below 4.5V. Do not apply more than 7V to
the EXTV_CC pin and ensure that EXTV_CC ≤ V_CC. The follow-
ing list summarizes the possible connections for EXTV_CC:
12
1. EXTV_CC grounded. INTV_CC is always powered from the
internal 5V regulator.
2. EXTV_CC connected to an external supply. A high effi-
ciency supply compatible with the MOSFET gate drive
requirements (typically 5V) can improve overall
efficiency.
3. EXTV_CC connected to an output derived boost network.
The low voltage output can be boosted using a charge
pump or flyback winding to greater than 4.7V. The system
will start-up using the internal linear regulator until the
boosted output supply is available.
External Gate Drive Buffers
The LTC3717 drivers are adequate for driving up to about
60nC into MOSFET switches with RMS currents of 50mA.
Applications with larger MOSFET switches or operating at
frequencies requiring greater RMS currents will benefit
from using external gate drive buffers such as the LTC1693.
Alternately, the external buffer circuit shown in Figure 4
can be used. Note that the bipolar devices reduce the
signal swing at the MOSFET gate, and benefit from an
increased EXTV_CC voltage of about 6V.
BOOST
INTV_CC
Q1
FMMT619
10Ω
TG
GATE
OF M1
Q2
FMMT720
Q3
FMMT619
10Ω
BG
GATE
OF M2
Q4
FMMT720
SW
PGND
3717 F04
Figure 4. Optional External Gate Driver
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3717 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3717 into a low quiescent current shutdown (I_Q <
30µA). Releasing the pin allows an internal 1.2µA current
source to charge up the external timing capacitor C_SS. If
RUN/SS has been pulled all the way to ground, there is a
delay before starting of about:
( ) ^t_DELAY
=
1.5V
1.2µA
C _SS
=
1.3s/µF
C _SS
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