TCM829ECT713 Datasheet, PDF (13/26 Pages) Microchip Technology – Switched Capacitor Voltage Converters

5.0 APPLICATIONS INFORMATION
5.0.1
OUTPUT VOLTAGE
CONSIDERATIONS
The TCM828/TCM829 devices perform voltage
conversion, but do not provide regulation. The output
voltage will droop in a linear manner with respect to
load current. The value of this equivalent output
resistance is approximately 25Ω nominal at +25°C and
VIN = +5V. VOUT is approximately – 5V at light loads,
and droops according to the equation below:
V_DROOP = I_OUT × R_OUT
V_OUT = –(V_IN –V_DROOP)
5.0.2 CHARGE PUMP EFFICIENCY
The overall power efficiency of the charge pump is
affected by four factors:
1. Losses from power consumed by the internal
oscillator, switch drive, etc. (which vary with
input voltage, temperature and oscillator
frequency).
2. I²R losses due to the on-resistance of the
MOSFET switches on-board the charge pump.
3. Charge pump capacitor losses due to effective
series resistance (ESR).
4. Losses that occur during charge transfer (from
the commutation capacitor to the output
capacitor) when a voltage difference between
the two capacitors exists.
Most of the conversion losses are due to factors 2, 3
and 4 above. These losses are shown in Equation 5-1.
EQUATION 5-1:
P_{LOSS(2, 3, 4 )}
=
I_O
U
2
T
×
R_OUT
≅
IO
2
UT
×
(^{---f--O-----S--1-C----})^{---C----1-- + 8RSWITCH + 4ESRC1 + ESRC2}
The 1/(f_OSC)(C1) term in Equation 5-1 is the effective
output resistance of an ideal switched capacitor circuit
(Figures 5-1 and 5-2).
TCM828/TCM829
The losses in the circuit due to factor 4 above are also
shown in Equation 5-2. The output voltage ripple is
shown in Equation 5-3.
EQUATION 5-2:
PLOSS₍₄₎
=
[ ( 0.5
)(C1
)(V
I
N
2
+
VO
U
2
T
)
+
(0.5
)
(C2
)
(VR
IPP
L
2
E
–
–2
VO
U
TVRIPPL
E
]
×
f
O
S
C
EQUATION 5-3:
V_RIPPLE= ₍---f--_O----_S^I--OC----^U₎--₍-^T--C----2----₎ + 2(I_OUT)(ESR_C2)
f
V⁺
C1
V_OUT
C2
R_L
FIGURE 5-1:
Model.
Ideal Switched Capacitor
R_EQUIV
V⁺
R_EQUIV= _f----×---1--C----1-
C2
V_OUT
R_L
FIGURE 5-2:
Resistance.
Equivalent Output
© 2010 Microchip Technology Inc.
DS21488B-page 13