LTC3608 Datasheet, PDF (15/24 Pages) Linear Technology – 18V, 8A Monolithic Synchronous Step-Down DC/DC Converter

LTC3608
APPLICATIONS INFORMATION
V_OUT2
C_SEC ⁺
1μF
V_OUT1
+
C_OUT
= PGND
= SGND
IN4148
SW
GND
40 39 38 37 36 35 34 33 32 31 30 29 28 27
T1
1:N
V_IN
+
C_IN
41 _SW
42 _SW
43 _SW
44 _SW
45
SW
46 _SW
47 _SW
48 _PVIN
49 _PVIN
50 _PVIN
51 _PVIN
52
PV_IN
LTC3608
SGND ²⁶
NC ²⁵
NC ²⁴
23
V_FB
22
I_ON
NC ²¹
SGND ²⁰
FCB ¹⁹
I_TH ¹⁸
V_RNG ¹⁷
PGOOD ¹⁶
SGND ¹⁵
R4
OPTIONAL EXTV_CC
CONNECTION
5V < V_OUT2 < 7V
R3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 ^{3608 F04}
SGND
SW
Figure 4. Secondary Output Loop and EXTV_CC Connection
within the LTC3608. The INTV_CC pin can supply up to 50mA
RMS and must be bypassed to ground with a minimum of
4.7μF tantalum or ceramic capacitor. Good bypassing is
necessary to supply the high transient currents required
by the MOSFET gate drivers.
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.
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_IN. The
following list summarizes the possible connections for
EXTV_CC:
1. EXTV_CC grounded. INTV_CC is always powered from the
internal 5V regulator.
2. EXTV_CC connectedtoanexternalsupply.Ahighefficiency
supply compatible with the MOSFET gate drive require-
ments (typically 5V) can improve overall efficiency.
Soft-Start and Latchoff with the RUN/SS Pin
The RUN/SS pin provides a means to shut down the
LTC3608 as well as a timer for soft-start and overcurrent
latchoff. Pulling the RUN/SS pin below 0.8V puts the
LTC3608 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
3608fa
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