LTC3245_15 Datasheet, PDF (11/18 Pages) Linear Technology – Wide VIN Range, Low Noise, 250mA Buck-Boost Charge Pump

LTC3245
Applications Information
2:1 Step-Down Charge Pump Operation
When the input supply is greater than about two times
the output voltage, the LTC3245 will operate in 2:1 step-
down mode. Charge transfer happens in two phases. On
the first phase the flying capacitor (C_FLY) is connected
between V_IN and V_OUT. On this phase C_FLY is charged up
and current is delivered to V_OUT. On the second phase the
flying capacitor (C_FLY) is connected between V_OUT and
GND. The charge stored on C_FLY during the first phase
is transferred to V_OUT on the second phase. When in 2:1
step-down mode the input current will be approximately
half of the total output current. The efficiency (η) and chip
power dissipation (P_D) in 2:1 are approximately:
η
≅
P_OUT
P_IN
=
V_OUT
V_IN •
• I_{OUT =}
1
2
I
OUT
2V_OUT
V_IN
P_D
=


V_IN
2
–
V_OUT


I
OUT
1:1 Step-Down Charge Pump Operation
When the input supply is less than about two times the
output voltage but more than the programmed output
voltage, the LTC3245 will operate in 1:1 step-down mode.
This method of regulation is very similar to a linear regula-
tor. Charge is delivered directly from V_IN to V_OUT through
most of the oscillator period. The charge transfer is briefly
interrupted at the end of the period. The interruption in
charge transfer improves stability and transient response.
When in 1:1 step-down mode the input current will be
approximately equal to the total output current. Thus
efficiency (η) and chip power dissipation (P_D) in 1:1 are
approximately:
η ≅ ^POUT = ^{VOUT • IOUT} = ^VOUT
P_IN V_IN • I_OUT V_IN
( ) P_D = V_IN– V_OUT I_OUT
1:2 Step-Up Charge Pump Operation
When the input supply is less than the output voltage the
LTC3245 will operate in 1:2 step-up mode. Charge trans-
fer happens in two phases. On the first phase the flying
capacitor (C_FLY) is connected between V_IN and GND. On
this phase C_FLY is charged up. On the second phase the
flying capacitor (C_FLY) is connected between V_IN and V_OUT
and the charge stored on C_FLY during the first phase is
transferred to V_OUT. When in 1:2 step-up mode the input
current will be approximately twice the total output cur-
rent. Thus efficiency (η) and chip power dissipation (P_D)
in 1:2 are approximately:
η
≅
P_OUT
P_IN
=
V_OUT
V_IN •
• I_{OUT =}
2 I_OUT
V_OUT
2V_IN
( ) P_D = 2V_IN– V_OUT I_OUT
Due to the limited drive in 1:2 step-up mode the device
always operates in Burst Mode operation when operating
at this conversion ratio. This is done to delay the onset of
dropout at the expense of more output ripple.
PGOOD Output Operation
The LTC3245 includes an open-drain power good (PGOOD)
output pin. If the chip is in shutdown or under UVLO con-
ditions (V_IN < 2.2V typical), PGOOD is low impedance to
ground. PGOOD becomes high impedance when V_OUT rises
to 95% (typical) of its regulation voltage. PGOOD stays
high impedance until V_OUT is shut down or drops below
the PGOOD threshold (91% typical) due to an overload
condition. A pull-up resistor can be inserted between
PGOOD and a low voltage positive logic supply (such as
V_OUT) to signal a valid power good condition. The use of
a large pull-up resistor on PGOOD and a capacitor placed
between PGOOD and GND can be used to delay the PGOOD
signal if desired.
V_OUT Ripple and Capacitor Selection
The type and value of capacitors used with the LTC3245
determine several important parameters such as regula-
tor control loop stability, output ripple and charge pump
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