TCM828ECT713 Datasheet, PDF (3/9 Pages) Microchip Technology – Switched Capacitor Voltage Converters

Switched Capacitor
Voltage Converters
TCM828
TCM829
DETAILED DESCRIPTION
The TCM828/829 charge pump converters invert the
voltage applied to the V_IN pin. Conversion consists of a two-
phase operation (Figure 1). During the first phase, switches
S2 and S4 are open and S1 and S3 are closed. During this
time, C1 charges to the voltage on V_IN and load current is
supplied from C2. During the second phase, S2 and S4 are
closed, and S1 and S3 are open. This action connects C1
across C2, restoring charge to C2.
S1
S2
IN
C1
TCM828/829
S3
S4
C2
V_OUT = – (V_IN)
Figure 1. Ideal Switched Capacitor Charge Pump
(4) Losses that occur during charge transfer (from the
commutation capacitor to the output capacitor)
when a voltage difference between the two capaci-
tors exists.
Most of the conversion losses are due to factors (2), (3)
and (4) above. These losses are given by Equation 1.
P_{LOSS (2, 3, 4)} = I_OUT2 x R_OUT
[ ] ^{≅ IOUT2 x}
1
(f_OSC) C1
+8R_SWITCH + 4ESR_C1 + ESR_C2
Equation 1.
The 1/(f_OSC)(C1) term in Equation 1 is the effective
output resistance of an ideal switched capacitor circuit
(Figures 2a, 2b).
The losses in the circuit due to factor (4) above
are also shown in Equation 2. The output voltage ripple is
given by Equation 3.
[ ^{PLOSS (4) = (0.5)(C1)(VIN2– VOUT2) + (0.5)(C2)(VRIPPLE2}
] ^{– 2VOUTVRIPPLE)} x ^fOSC
Equation 2.
APPLICATIONS INFORMATION
Output Voltage Considerations
The TCM828/829 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 V_IN = +5V. V_OUT is approximately – 5V at light
loads, and droops according to the equation below:
V_DROOP = I_OUT x R_OUT
V_OUT = – (V_IN – V_DROOP)
V_RIPPLE =
I_OUT
(f_OSC)(C2)
+2(I_OUT)(ESR_C2)
Equation 3.
f
V⁺
V_OUT
C1
C2
R_L
Charge Pump Efficiency
The overall power efficiency of the charge pump is
affected by four factors:
(1) Losses from power consumed by the internal oscil-
lator, switch drive, etc. (which vary with input volt-
age, 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).
© 2001 Microchip Technology Inc. DS21488A
3
Figure 2a. Ideal Switched Capacitor Model
R_EQUIV
V⁺
R_{EQUIV =} 1
f x C1
C2
V_OUT
R_L
Figure 2b. Equivalent Output Resistance
TCM828/829-4 5/22/00