LTC3252_15 Datasheet, PDF (10/12 Pages) Linear Technology – Dual, Low Noise, Inductorless Step-Down DC/DC Converter

LTC3252
U
OPERATIO (Refer to Simplified Block Diagram)
Thermal Management
To reduce the maximum junction temperature, a good
thermal connection to the PC board is recommended.
Soldering the exposed paddle of the IC to the PCB and
maintaining a solid ground plane under the device on one
or more layers of the PC board, the thermal resistance of
the package can be as small as 40°C/W. By applying the
suggested thermal management techniques the IC junc-
tion temperature should never exceed 125°C even under
worst case operating conditions.
Power Efficiency
The power efficiency (η) of the LTC3252 is approximately
50% higher than a conventional linear regulator. This
occurs because the input current for a 2-to-1 step-down
charge pump is approximately half the output current. For
an ideal 2-to-1 step-down charge pump the power effi-
ciency is given by:
η≡
P_OUT
P_IN
=
V_OUT •I_OUT
V_IN
•
1
2
I_OUT
=
2V_OUT
V_IN
The switching losses and quiescent current of the LTC3252
are designed to minimize efficiency loss over the entire
output current range, causing only a couple % error from
the theoretical efficiency. For example with V_IN = 3.6V,
R_A
R_B
R_B'
R_A'
EN1
V_IN
C_IN
C1
1
LTC3252
EN2
C2
I_OUT1 = 150mA and OUT1 regulating at 1.5V the measured
efficiency is 80.6% which is in close agreement with the
theoretical 83.3% calculation.
Programming the LTC3252 Output Voltages (FB1 and
FB2 Pin)
Each output of the LTC3252 is programmed to an arbitrary
voltage via an external resistive divider. Figure 7 shows the
required voltage divider connection. The voltage divider
ratio is given by the expression:
R_{A =} OUT _{− 1}
R_B 0.8V
Typical values for total voltage divider resistance can
range from several kΩs up to 1MΩ.
The user may want to consider load regulation when
setting the desired output voltage. The closed loop output
impedance of the LTC3252 is approximately:
R_O
=
0.08Ω
•
OUT
0.8V
For a 1.5V output, R_O is 0.15Ω, which produces a 38mV
output change for a 250mA load current step. Thus, the
user may want to target an unloaded output voltage
slightly higher than desired to compensate for the output
load conditions. The output may be programmed for
regulation voltages of 0.9V to 1.6V.
Since the LTC3252 employs a 2-to-1 charge pump archi-
tecture, it is not possible to achieve output voltages
greater than half the available input voltage. The minimum
V_IN supply required for regulation can be determined by
the following equation:
V_IN (MIN) ≤ 2 • (V_OUT (MIN) + I_OUT • R_OL)
OUT1
C_O1
OUT2
C_O2
GND
(CONNECT DIRECTLY TO GROUND PLANE)
3252 F06
Figure 6. Suggested Layout for the LTC3252
10
OUT1
R_{A =} OUT1 _{– 1}
R_B 0.8V
C_O1
OUT1 OUT2
OUT2
R_A
LTC3252
R_A'
R_A' ₌ OUT2 _{– 1}
FB1
FB2
C_O2 ^{RB' 0.8V}
R_B
R_B'
GND
GND
3252 F07
Figure 7. Programming the LTC3252
3252f