LTC3620 Datasheet, PDF (11/16 Pages) Linear Technology – Ultralow Power 15mA Synchronous Step-Down Switching Regulator

LTC3620
APPLICATIONS INFORMATION
Setting Output Voltage
The output voltage is set by tying V_FB to a resistive divider
using the following formula (refer to Figure 3):
V_OUT
=
0.6V
(R1+
R2
R2)
R1 and R2 should be large to minimize standing load
current and improve efficiency.
The fixed output version, the LTC3620-1, includes an
internal resistive divider, eliminating the need for external
resistors. The resistor divider is chosen such that the V_FB
input current is approximately 1μA. For this version, the
V_FB pin should be connected directly to V_OUT.
Maximum Load Current and Maximum Frequency
The maximum current that the LTC3620 can provide is
calculated to be just slightly less than half the maximum
peak current.
The inductor value will determine how much energy is
delivered to the output for each switching cycle, and thus
the duration of each pulse and the maximum frequency.
Larger inductors will have slower ramp rates, longer pulses,
and thus lower maximum frequencies. Conversely, smaller
inductors will result in higher maximum frequencies.
When using a frequency clamp, large abrupt increasing
load steps from levels below the locking range to levels
near the maximum output may result in a large drop in
the output voltage. This is due to the low bandwidth of
the frequency clamp loop in returning the peak inductor
current to its maximum.
Thermal Considerations
The LTC3620 requires the package backplane metal to be
well soldered to the PC board. This gives the DFN package
exceptional thermal properties, making it difficult in normal
operation to exceed the maximum junction temperature
of the part. In most applications the LTC3620 does not
dissipate much heat due to its high efficiency and low
current. In applications where the LTC3620 is running at
high ambient temperatures and high load currents, the heat
dissipated may exceed the maximum junction temperature
of the part if it is not well thermally grounded.
Design Example
This example designs a 1.1V output using a Li-Ion bat-
tery with voltages between 2.8V to 4.2V, and an average
of 3.6V. The internally provided 50kHz clock will be used
for the minimum switching frequency, so the FMIN/MODE
pin will be pulled low. For a 1.1V output, an 18μH inductor
should be used (refer to Table 2).
C_OUT can be chosen from Table 2 or can be based on a
desired maximum output voltage ripple, ΔV_OUT. For this
case let’s use a maximum ΔV_OUT equal to 1% of V_OUT,
or 11mV.
( ) ^{35mA2 (22µH)(3.6V)}
C_OUT = _{2ΔVOUT (1.1V)(3.6V – 1.1V)} = 1.6µF ≈ 1.5µF
V_IN
2.9V TO 5.5V
V_IN
RUN LOBATB
LTC3620
1μF
SW
CER
LOBATB
1M
L
22pF
V_OUT
1.1V
FMIN/MODE V_FB
GND
R1
R2
C_OUT
3620 F03
Figure 3. Design Example Schematic
3620f
11