LTC3543 Datasheet, PDF (10/16 Pages) Linear Technology – 600mA Synchronous Step Down Buck Regulator with PLL, Soft-Start and Spread Spectrum

LTC3543
APPLICATIO S I FOR ATIO
Table 1. Representative Surface Mount Inductors
MANUFACTURER
PART NUMBER
VALUE (µH)
TDK
VLF3010AT-2R2M1R0
2.2
VLF3012AT-2R2M1R0
2.2
VLCF4020T-2R2N1R7
2.2
VLCF5020-2R7N1R7
2.7
VLCF5020-3R3N1R6
3.3
VLCF5020-4R7N1R4
4.7
Sumida
CDRH2D18/HP-2R2NC
2.2
Taiyo Yuden
NR4018T4R7M
4.7
NP03SB4R7M
4.7
CoEv
DN4835-2R2
2.2
DN4835-3R3
3.3
DN4835-6R8
6.8
muRata
LQH32CN2R2M33
2.2
LQH55DN2R2M03
2.2
LQH55DN3R3M03
3.3
LQH55DN4R7M03
4.7
MAX DC CURRENT (A)
1.0
1.0
1.7
1.7
1.6
1.4
1.6
1.7
1.2
2.6
2.43
1.41
0.79
3.2
2.9
2.7
DCR (mΩ)
100
88
54
58
69
79
48
90
47
47
58
117
97
29
36
41
HEIGHT (mm)
1.0
1.2
2.0
2.0
2.0
2.0
2.0
1.8
1.8
3.5
3.5
3.5
3.2
4.7
4.7
4.7
This formula has a maximum of V_IN = 2V_OUT, where I_RMS
= I_OUT/2. This simple worst-case condition is commonly
used for design because even significant deviations do not
offer much relief. Note that the capacitor manufacturer’s
ripple current ratings are often based on 2000 hours of
life. This makes it advisable to further derate the capaci-
tor, or choose a capacitor rated at a higher temperature
than required. Always consult the manufacturer if there
are any questions.
The selection of C_OUT is driven by the required effective
series resistance (ESR). Typically, once the ESR require-
ment for C_OUT has been met, the RMS current rating
generally far exceeds the I_RIPPLE(P-P) requirement. The
output ripple ΔV_OUT is determined by:
∆V_OUT
≅
∆I_L
⎛
⎝⎜
ESR
+
1
8 fC_OUT
⎞
⎠⎟
(3)
where f = operating frequency, C_OUT = output capacitance
and ΔI_L = ripple current in the inductor. For a fixed output
voltage, the output ripple is highest at maximum input
voltage since ΔI_L increases with input voltage.
Aluminum electrolytic and dry tantalum capacitors are both
available in surface mount configurations. In the case of
10
tantalum, it is critical that the capacitors are surge tested
for use in switching power supplies. An excellent choice
is the AVX TPS series of surface mount tantalum. These
are specially constructed and tested for low ESR so they
give the lowest ESR for a given volume. Other capacitor
types include the Sanyo POSCAP, the Kemet T510 and T495
series, and the Sprague 593D and 595D series. Consult
the manufacturer for other specific recommendations.
C1 Selection
When spread spectrum operation is enabled, the fre-
quency of the LTC3543 is randomly varied over the
range of frequencies between 2MHz and 3MHz. In
this case, a capacitor should be connected between
the CAP pin and GND to smooth out the changes in fre-
quency. This not only provides a smoother frequency spec-
trum but also ensures that the switching regulator remains
stable by preventing abrupt changes in frequency.
When the PLL mode is enabled, if the external clock fre-
quency is greater than the internal oscillator’s frequency
(OSC), then current is sourced continuously, pulling up the
voltage on the CAP pin. If the external clock frequency is
less than OSC, current is sunk continuously, pulling down
the voltage on the CAP pin. When the external and internal
3543f