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LTC3226

3226fa

APPLICATIONS INFORMATION

where VF is the forward voltage of the diode. As an ex-

ample, if RPF1 = 200k, RPF2 = 120k, RH = 2M, and VF =

0.4V, the additional hysteresis provided by this network

can be calculated using the above equation as follows:

ΔV

IN(HYS) = 1.2 + 0.02 – 0.4

()V•

200k

= 82mV

GND is needed to set VOUT and the reset threshold 7.5%

below the VOUT programmed voltage. However, the reset

threshold can be set independent of VOUT by an additional

resistor divider.

Effective Open-Loop Output Resistance (ROL) of the

Charge Pump

The effective open-loop output resistance (ROL) of a charge

pump determines the strength of a charge pump. The value

of this parameter depends on many factors such as the

oscillator frequency (fOSC), value of the flying capacitor

(CFLY), the nonoverlap time, the internal switch resistances

(RS), and the ESR of the external capacitors. A first order

approximation of ROL is given below:

OL ≅ 2 Σ

=1to4

S +

OSC •CFLY

For the LTC3226 charge pump, the sum of the switch

resistances is approximately 2.5Ω in a typical applica-

tion where VIN = 3.3V and VCPO = 5V. For CFLY = 1μF and

fOSC = 1MHz, the effective open-loop output resistance

of the charge pump can be approximated from the above

equation as follows:

OL ≅ 2 • 2.5Ω+

1MHz • 1µF

= 6Ω

Maximum Available Charge Current

In the absence of any internal current limit, the maximum

available current out of a charge pump in 2x mode can be

calculated from the charge pump input and output voltage

and the effective open-loop output resistance ROL using

the following equation:

CHRG =

IN –VCPO

For example, if the LTC3226 charge pump (ROL ≅ 6Ω)

has to charge a supercapacitor to 5V from 2.5V input,

the charge current available when VCPO = 4.8V can be

calculated as follows:

CHRG =

2 • 2.5V – 4.8V

= 33.3mA

3226 F02

LTC3226

VIN

VOUT

PFO

PFI

470k

RPF1

RPF2

Figure 2. Increasing PFI Comparator Hysteresis Externally

Programming the LDO Output Voltage (VOUT)

The LDO output voltage in backup mode can be pro-

grammed for any voltage between 2.5V to 5.3V by using a

resistor divider from the VOUT pin to GND via the LDO_FB

pin such that:

OUT = VLDO _ FB •1+

FB1

FB2

⎛

⎝⎜

⎞

⎠⎟

where VLDO_FB is 0.8V. See the Block Diagram in Figure 1.

Typical values for RFB1 and RFB2 are in the range of 40k to

500k. Too small a resistor will result in a large quiescent

current whereas too large a resistor coupled with LDO_FB

pin capacitance will create an additional pole and may

cause loop instability.

Programming the Reset Threshold

The threshold for the reset comparator can be programmed

by using a resistor divider from the VOUT pin to GND via

the RST_FB pin such that:

OUT = VRST _ FB •1+

FB1

FB2

⎛

⎝⎜

⎞

⎠⎟

where VRST_FB is 0.74V. See Figure 1. Typical values for

RFB1 and RFB3 are in the range of 40k to 5M. In most ap-

plications, the LDO_FB and RST_FB pins can be shorted

together and only one resistor divider between VOUT and

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