TC1240 Datasheet, PDF (3/11 Pages) Microchip Technology – Positive Doubling Charge Pump with Shutdown in SOT Package

Positive Doubling Charge Pump with
Shutdown in SOT Package
TC1240
DETAILED DESCRIPTION
The TC1240 charge pump converter doubles the volt-
age 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.
During this second phase, C1 is level shifted upward by
V_IN volts. This connects C1 to the reservoir capacitor C2,
allowing energy to be delivered to the output as needed. The
actual voltage is slightly lower than 2 x V_IN since the four
switches (S1 - S4) have an on-resistance and the load drains
charge from reservoir capacitor C2.
S1
S2
V_IN
C1
S3
S4
TC1240
OUT = 2 x V_IN
C2
V_IN
OSC
Figure 1. Ideal Swiched Capacitor Charge Pump Doubler
APPLICATIONS INFORMATION
Output Voltage Considerations
TheTC1240 performs voltage doubling but does 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 17Ω nominal
at +25°C and V_IN = +2.8V. V_OUT is approximately +5.6V at
light loads, and droops according to the equation below:
(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
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) and
(3) above. These losses are given by Equation 1(b).
(a) P_{LOSS (2, 3)} = I_OUT ² x R_OUT
[ ] ^{(b) ≅ IOUT2 x}
1
(f_PUMP)
C1+8R^SWITCH
+
4ESR_C1
+
ESR_C2
Equation 1.
The pump frequency in Equation 1(b) is defined as one-
half the oscillator frequency (i.e. f_PUMP = f_OSC/2). The
1/(f_PUMP)(C1) term in Equation 1(b) is the effective output
resistance of an ideal switched capacitor circuit (Figures 2a,
2b). The value of R_SWITCH can be approximated at 1.4Ω for
the TC1240.
The remaining losses in the circuit are due to factor (4)
above, and are shown in Equation 2. The output voltage
ripple is given by Equation 3.
P_LOSS(4) = [(0.5)(C1) (4V_IN2– V_OUT2 ) + (0.5)(C2)(2V_OUT
V_RIPPLE – V_RIPPLE2 )] x f_OSC
Equation 2.
V_RIPPLE = I_OUT +2(I_OUT)(ESR_C2)
(f_OSC)(C2)
Equation 3.
f
V⁺
V_OUT
C1
C2
R_L
V_DROOP = I_OUT x R_OUT
V_OUT = 2 x V_IN – V_DROOP
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 voltage,
temperature and oscillator frequency).
TC1240-1 7/7/00 DS21333A
3
Figure 2a. Ideal Swiched Capacitor Model
R_EQUIV
V⁺
R_{EQUIV =} 1
f x C1
C2
V_OUT
R_L
Figure 2b. Equivalent Output Resistance
© 2001 Microchip Technology Inc.