TC115_13 Datasheet, PDF (7/14 Pages) Microchip Technology – PFM/PWM Step-Up DC/DC Converter

TC115
Rewriting in terms of input and output currents and volt-
ages:
EQUATION 5-5:
V_INMINI_INMAX
=
_---V_{----O---U----T---M---A--X---}_--_---I_{--O----U---T---M---A--X---}
Efficiency
Solving for input current:
EQUATION 5-6:
I_INMAX = -_--_E-^V---_f-^O-_f--_i^U--_c--^T-_i--^M_e---^A_n--^X-_c--^--_y^--^I-_--^O_--_V-^U---_I-^T-_N--^M--_M-^A--_IX-_N--^-_
The sawtooth current is centered on the DC current
level, swinging equally above and below the DC current
calculated in Equation 5-6. The peak inductor current is
the sum of the DC current plus half the ac current. Note
that minimum input voltage should be used when
calculating the ac inductor current (Equation 5-9).
EQUATION 5-7:
V_L = Ldi  dt
EQUATION 5-8:
di = V_Ldi  dt
EQUATION 5-9:
Where:
di = ^---^V----^I--^N---^M---^I-^N----^–-----^V---^S---^W-----^--^t--^O---^N--
L
V_SW = The voltage drop across the internal
N-channel MOSFET.
Combining the DC current calculated in Equation 5-6,
with half the peak ac current calculated in Equation 5-9,
the peak inductor current is given by:
EQUATION 5-10:
I_PK = I_INMAX + 0.5di
5.3 Internal Transistor Switch
The LX pin has a typical ON resistance of 1.4.
Therefore, peak switch current is given by (V_IN/1.4).
The internal transistor switch has a maximum design
rating of 350 mA. An oscillator frequency doubling cir-
cuit is an included guard against high switching cur-
rents. Should the voltage on the LX pin rise above 1.3V
(max) while the internal N-channel switch is ON, the
oscillator frequency automatically doubles to minimize
ON time. Although reduced, switch current still flows
because the PWM remains in operation. Therefore, the
LX input is not internally current-limited and care must
be taken never to exceed the 350 mA maximum limit.
Failure to observe this will result in damage to the
regulator.
5.4 Output Capacitor
The effective series resistance of the output capacitor
directly affects the amplitude of the output voltage
ripple (The product of the peak inductor current and the
ESR determines output ripple amplitude). Therefore, a
capacitor with the lowest possible ESR should be
selected. Smaller capacitors are acceptable for light
loads (or in applications where ripple is not a concern).
The Sprague^® 595D series of tantalum capacitors are
among the smallest of all low ESR surface mount
capacitors available. Table 5-1 lists suggested
components and suppliers.
5.5 Board Layout Guidelines
As with all inductive switching regulators, the TC115
generates fast switching waveforms which radiate
noise. Interconnecting lead lengths should be
minimized to keep stray capacitance, trace resistance
and radiated noise as low as possible. In addition, the
GND pin, input bypass capacitor and output filter
capacitor ground leads should be connected to a single
point.
TABLE 5-1: SUGGESTED COMPONENTS AND SUPPLIERS
Type
Surface Mount
Through-Hole
Inductors
Sumida^®
CD54 Series
CDR125 Series
Coiltronics™
CTX Series
Sumida^®
RCH855 Series
RCH110 Series
Renco^®
RL1284-12
Capacitors
Matsuo^®
267 Series
Sprague^®
595D Series
Nichicon™
F93 Series
Sanyo™
OS-CON Series
Nichicon™
PL Series
Diodes
Nihon
EC10 Series
Matsushita™
MA735 Series
ON Semiconductor^®
1N5817 - 1N5822
 2002-2012 Microchip Technology Inc.
DS21361D-page 7