LTC3632_15 Datasheet, PDF (15/22 Pages) Linear Technology – High Efficiency, High Voltage 20mA Synchronous Step-Down Converter

LTC3632
APPLICATIONS INFORMATION
The gate charge current results from switching the gate
capacitance of the internal power MOSFET switches.
Each time the gate is switched from high to low to
high again, a packet of charge, dQ, moves from V_IN to
ground. The resulting dQ/dt is the current out of V_IN
that is typically larger than the DC bias current.
2. I²R losses are calculated from the resistances of the
internal switches, R_SW, and external inductor R_L. When
switching, the average output current flowing through
the inductor is “chopped” between the high side PMOS
switch and the low side NMOS switch. Thus, the series
resistance looking back into the switch pin is a function
of the top and bottom switch R_­DS(ON) values and the
duty cycle (DC = V_OUT/V_­IN) as follows:
R_SW = (R_DS(ON)TOP)DC + (R_DS(ON)BOT)(1 – DC)
The R_DS(ON) for both the top and bottom MOSFETs can
be obtained from the Typical Performance Characteris-
tics curves. Thus, to obtain the I²R losses, simply add
R_SW to R_L and multiply the result by the square of the
average output current:
I²R Loss = I_O2(R_SW + R_L)
Other losses, including C_IN and C_OUT ESR dissipative
losses and inductor core losses, generally account for
less than 2% of the total power loss.
Thermal Considerations
The LTC3632 does not dissipate much heat due to its high
efficiency and low peak current level. Even in worst-case
conditions (high ambient temperature, maximum peak
current and high duty cycle), the junction temperature will
exceed ambient temperature by only a few degrees.
Design Example
As a design example, consider using the LTC3632 in an
application with the following specifications: V_IN = 24V,
V_OUT = 3.3V, I_OUT = 20mA, f = 250kHz. Furthermore, as-
sume for this example that switching should start when
V_IN is greater than 12V and should stop when V_IN is less
than 8V.
First, calculate the inductor value that gives the required
switching frequency:
L
=


3.3V 
250kHz • 50mA ^
•


1–
3.3V 
24V ^
≅
220µH
Next, verify that this value meets the L_MIN requirement.
For this input voltage and peak current, the minimum
inductor value is:
L_MIN
=
24V • 100ns
50mA
≅
48µH
Therefore, the minimum inductor requirement is satisfied,
and the 220μH inductor value may be used.
Next, C_IN and C_OUT are selected. For this design, C_IN should
be size for a current rating of at least:
I_RMS
=
20mA
•
3.3V
24V
•
24V
3.3V
–
1
≅
7mA_RMS
Due to the low peak current of the LTC3632, decoupling
the V_IN supply with a 1µF capacitor is adequate for most
applications.
C_OUT will be selected based on the output voltage ripple
requirement. For a 1% (33mV) output voltage ripple at no
load, C_OUT can be calculated from:
C_OUT
=
50mA • 4 • 10 ^–6

2_33mV

–
3.3V
160



An 8.1µF capacitor gives this typical output voltage ripple at
no load. Choose a 10µF capacitor as a standard value.
The output voltage can now be programmed by choosing
the values of R1 and R2. Choose R2 = 240k and calculate
R1 as:
R1=



V_OUT
0.8V
–

1_

•
R2
=
750k
3632fc
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