Improved Charge Pump

In this section the cross-coupled charge pump is discussed in detail. The shoot-though current loss may occur in a cross-coupled structure. The shoot-though current will lead to the reduction of voltage gain and efficiency. The shoot-though current here is the current flow from output to input. For a charge pump which boosts the input voltage to a higher level, the current should flow from input supply to output node. Therefore the shoot-though current is a reversion loss which we do not want it to appear. An improve structure of the cross-coupled charge pump will be proposed to reduce the shoot-though current loss. The more the shoot-though current loss can be reduced, the higher gain and efficiency can be achieved.

3.2.1 Shoot-Though Current Loss

In the cross-coupled charge pump such as the Favrat charge pump, there is a loss called shoot-though current loss. It is due to the control of the pass transistors during switching. As shown in Fig. 27, the voltage transition at the node V₁ and V₂ can not be controlled during switching. The shoot-though currents will occur on pass transistors ML1, ML2, MR1 and MR2

which are controlled by node V₁ and V₂ during switching. The currents I₁ and I₂ are the shoot-through currents which arise in pass transistors ML1 and MR1. And the currents I3 and I4

are the shoot-through currents which arise in pass transistors M_L2 and M_R2. Because the cross-coupled charge pump is a symmetric structure, therefore the shoot-through current is generated each half cycle in similar way.

V^DD

Fig. 27 Shoot-though current generation mechanism in Favrat charge pump

Fig. 27 shows the shoot-through current generation mechanism. The voltages V1 and V2

produce shoot-through current in four different ways. Voltages V₁ and V₂ are the control of the pass transistors ML1, ML2, MR1 and MR2 and they are switched between VDD and 2VDD. Let’s see the generation of the soot-through current I₁, it is due to the leakage from pass transistor ML1. During the switching where V1 increased from VDD to 2VDD and V2 decreased from 2V_DD to V_DD, at the transition in the initial phase of clock signal where

2 1 tn 1 DD

V − ≥ V V and V > V

Vtn is the threshold voltage of pass transistors ML1 and MR1. Under this condition, the first shoot-through current occurs and flow from node 1 through M_L1 to the input power supply VDD. The second shoot-through current is generated in a similar way as the symmetry of the cross-coupled structure. Shoot-through current I₂ flows from node 2 through M_R1 to the input power supply voltage when

1 2 tn 1 DD

V − V ≥ V and V > V

AS shown in Fig. 27, the third and fourth shoot-through currents leak from output node the input power supply. This occurs when pass transistor pairs M_L1 and M_L2, M_R1 and M_R2 are simultaneously conducted, respectively.

V

Fig. 28 The occurrence of the shoot-though current during switching

As shown in Fig. 28, The duration of the shoot-through currents I1 and I2 are the blue regions. And the red regions are the duration that shoot-through currents I₃ and I₄ occur. I₃ occurs when ML1 and ML2 are simultaneously conducted. ML2 is turned on when

2

2

2

V ≤ V − V and V > V

Based on equation (14), the minimum input power supply voltage V_DD to turned on transistor ML1 during the turning on of the transistor ML2 is given by

( )

max 2 , 2 2

DD tp tp tn tp tn

V ≥ V V + V = V + V

The red regions where simultaneous conduction occurs in Fig. 28 increases with the input supply voltage VDD. Therefore, the higher the input supply voltage is, the larger the region of simultaneous conduction is. It will lead to larger power loss and poor power efficiency.

3.2.2 Proposed Charge Pump with Loss Reduction Technique

From the discussion in previous section, we want to develop an efficient structure of cross-coupled charge pump to reduce the shoot-through current during switching and the reversion loss. The proposed cross-coupled charge pump circuit is shown in Fig. 29.

V^DD

Fig. 29 Proposed cross-coupled charge pump with loss reduction technique

As shown in Fig. 29, in the proposed structure, two transistors (M_L4, M_R4) and two resistors (R1, R2) are added into the circuit. Transistors ML4 and MR4 are controlled by an extra level shifter to ensure the proper turning on and off. The principle of operation of the proposed cross-coupled charge pump is going to be unfolded in this paragraph. In the proposed cross-coupled charge pump, M_L4 is turned on when M_L2 is turned off and M_R4 is turned on when MR2 is turned off. The transistors ML4 and MR4 are used to increase the transition speed of the control signal V₁ and V₂ turning from V_DD to 2V_DD. It provides an

additional current path to charge the gate terminals of the power transistors ML2 and MR2 and makes them turning off much faster. The resistors R₁ and R₂ are added to slower the transition speed of the gate if power transistor ML2 and MR2. It makes power transistors to turn on much slower due to the additional RC delay.

V

2V

V

V

2V

-Vtp M

ON M

ON

M

ON

M

ON

Fig. 30 Simultaneous transitions of control voltages in the proposed structure

The simultaneous transitions of the control clocking signal V1 and V2 is shown in Fig.

30. And the control clocking signal V1 and V2 of the proposed cross-coupled charge pump is the blue line in Fig. 30. The gate of M_L2 is connected to V₂ through R₁. While V2 is changed form 2VDD to VDD slower, ML2 will be turned on slower. Therefore the ML1 on will not overlap to the on time of M_L2. This creates a break-before-make mechanism which will prevent the two transistors from simultaneous conduction. The gate of MR2 is connected to V1

through R₂. During the switching of V₁ from V_DD to 2 V_DD, because the charging assistant current created by MR4 will fasten the speed from low to high, power transistor MR2 will be turned off quickly. It also creates a break-before-make and prevents M_R1 and M_R2 from simultaneous conduction. As mentioned above, the shoot-through current can be reduced. The

following guidelines. The resistors need to be design large enough to let the slower transition from 2V_DD to V_DD of V₂ can make sure that M_L1 is turned on after M_L2 is turned off. And the size of the additional transistors are big enough to let the faster transition from VDD to 2VDD

of V₁ can make sure that M_R2 is turned off before M_R1 is turned on.