LTC3620 Datasheet, PDF (9/16 Pages) Linear Technology – Ultralow Power 15mA Synchronous Step-Down Switching Regulator

LTC3620
APPLICATIONS INFORMATION
Choosing an Inductor
There are a number of different values, sizes and brands
of inductors that will work well with this part. Table 1 has a
number of recommended inductors, though there are many
other manufacturers and devices that may also be suitable.
Consult each manufacturer for more detailed information
and for their entire selection of related parts.
Table 1: Representative Surface Mount Inductors
VENDOR
PART
NUMBER
MAX DC
VALUE
CURRENT
(μH) DCR (Ω) (mA)
W×L×H
(mm³)
Taiyo
CBMF1608T 22 ±10% 1.3 Max 70 0.8 × 1.6 × 0.8
Yuden
Murata LQH2MC_02 18 ±20% 1.8 ±30% 190
22 ±20% 2.1 ±30% 185
1.6 × 2 × 0.9
Würth 744028220 22 ±30% 1.48 Max 270 2.8 × 2.8 × 1.1
Electronics
Coilcraft
LPS3010 18 ±20% 1.0 Max
22 ±20% 1.2 Max
380 2.95 × 2.95 × 0.9
320
There is a trade-off between physical size and efficiency;
The inductors in Table 1 are shown because of their small
footprints, choose larger sized inductors with less core
loss and lower DCR to maximize efficiency.
The ideal inductor value will vary depending on which
characteristics are most critical to the designer. Use the
equations and recommendations in the next sections to help
you find the correct inductance value for your design.
Avoiding Audio Range Switching
In order to best avoid switching in the audio range at the
lowest possible load current, the minimum frequency
should be set as low as is acceptable, and the inductor
value should be minimized. For a 1.1V output the smallest
recommended inductor value is 15μH.
Adjusting for V_OUT
The inductor current peak and zero crossing are dependent
on the dI/dt. The equations for the rising and falling slopes
are as follows:
Rising dI/dt = (V_IN-V_OUT)/L
Falling dI/dt = V_OUT/L
The part is optimized to get 35mA peaks for V_IN = 3.6V and
V_OUT = 1.1V with an 18μH inductor. If the falling slope is
too steep the NFET will continue to conduct shortly after
the inductor current reaches zero, causing a small reverse
current. This means the net power delivered with every
pulse will decrease. To mitigate this problem the inductor
can be resized. Table 2 shows recommended inductors and
output capacitors for commonly used output voltages.
Table 2. Recommended Inductor and Output Capacitor Sizes for
Different V_OUT
V_OUT (V)
0.9
L (μH)
15
C_OUT (μF)
2.2
1.1
22
1
1.1 (LTC3620-1)
22
2.2
1.8
33
2.2
2.5
47
4.7
Because the rising dI/dt decreases for increased V_OUT
and increased L, the inductor current peaks will decrease,
causing the maximum load current to decrease as well.
Figure 2 shows typical maximum load current versus
output voltage.
20
T_A = 25°C
19
18
17
16
15
14
13
12
11
10
0.6
1.1
1.6
2.1
OUTPUT VOLTAGE (V)
2.6
3620 F02
Figure 2. Maximum Output Current vs V_OUT, V_IN = 3.6V
Output Voltage Ripple
The quantity of charge transferred from V_IN to V_OUT per
switching cycle is directly proportional to the inductor
value. Consequently, the output voltage ripple is directly
proportional to the inductor value, and the switching
frequency for a given load is inversely proportional to the
inductor value. For a given load current, higher switching
frequency will typically lower the efficiency because of the
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