LTC1649 Datasheet, PDF (10/16 Pages) Linear Technology – 3.3V Input High Power Step-Down Switching Regulator Controller

LTC1649
APPLICATIONS INFORMATION
tors that work well in LTC1649 applications. A common
way to lower ESR and raise ripple current capability is to
parallel several capacitors. A typical LTC1649 application
might require an input capacitor with a 5A ripple current
capacity and 2% output shift with a 10A output load step,
which requires a 0.005Ω output capacitor ESR. Sanyo OS-
CON part number 10SA220M (220µF/10V) capacitors
feature 2.3A allowable ripple current at 85°C and 0.035Ω
ESR; three in parallel at the input and seven at the output
will meet the above requirements.
Input Supply Considerations/Charge Pump
The LTC1649 requires four supply voltages to operate:
V_IN, V_CC, PV_CC1 and PV_CC2. V_IN is the primary high power
input, supplying current to the drain of Q1 and the input to
the internal charge pump at the V_IN pin. This supply must
be between 2.7V and 6V for the LTC1649 to operate
properly. An internal charge pump uses the voltage at V_IN
to generate a regulated 5V output at CP_OUT. This charge
pump requires an external 1µF capacitor connected be-
tween the C ⁺ and C ^– pins, and an external 10µF reservoir
capacitor connected from CP_OUT to ground. The voltage at
CP_OUT must always be greater than or equal to V_IN. If V_IN
is expected to rise above 5V, an additional Schottky diode
(D5) should be added from V_IN to CP_OUT.
CP_OUT is typically connected to PV_CC2 directly, providing
the 5V supply that the G2 driver output uses to drive Q2.
PV_CC2 requires a 10µF bypass to ground; this capacitor
can double as the CP_OUT reservoir capacitor, allowing a
typical application with CP_OUT and PV_CC2 connected to-
gether to get away with only a single 10µF capacitor at this
node, located close to the PV_CC2 pin. V_CC can also be
powered from CP_OUT, but is somewhat sensitive to noise.
PV_CC2 happens to be a significant noisemaker, so most
applications require an RC filter from CP_OUT/PV_CC2 to V_CC.
22Ω and 10µF are typical filter values that work well in
most applications.
PV_CC1 needs to be boosted to a level higher than CP_OUT to
provide gate drive to Q1. The LTC1649 initially used a
charge pump from V_IN to create CP_OUT; the typical appli-
cation uses a second charge pump to generate the PV_CC1
supply. This second charge pump consists of a Schottky
diode (D_CP) from CP_OUT to PV_CC1, and a 1µF capacitor
from PV_CC1 to the source of Q1. While Q2 is on, the diode
charges the capacitor to CP_OUT. When Q1 comes on, its
source rises to V_IN, and the cap hauls PV_CC1 up to (CP_OUT
+ V_IN), adequate to fully turn on Q1. When Q1 turns back
off, PV_CC1 drops back down to CP_OUT; fortunately, we’re
not interested in turning Q1 on at this point, so the lower
voltage doesn’t cause problems. The next time Q1 comes
on, PV_CC1 bounces back up to (CP_OUT + V_IN), keeping Q1
happy. Figure 4 shows a complete power supply circuit for
the LTC1649.
*OPTIONAL
D5*
FOR V_IN ≥ 5V
22Ω
+
10µF
D_CP
+
10µF
V_IN
+
C_IN
1µF
V_IN
CP_OUT
V_CC
PV_CC2
C⁺
1µF
CHARGE
PUMP
C^–
DRIVE
CIRCUITRY
PV_CC1
G1
G2
Q1
L1
V_OUT
+
Q2
C_OUT
LTC1649
1649 F04
Figure 4. LTC1649 Power Supplies
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