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V

R

(DPPM)

SF

(1)

Case 2: USB Mode (Mode = L)

bq24070

SLUS694A – MARCH 2006 – REVISED MARCH 2006

FUNCTIONAL DESCRIPTIONS (continued)

1. This feature allows the designer to select a lower power wall adapter, if the average system load is

moderate compared to its peak power. For example, if the peak system load is 1.75 A, average system load

is 0.5 A and battery fast-charge current is 1.25 A, the total peak demand could be 3 A. With DPPM, a 2-A

adaptor could be selected instead of a 3.25-A supply. During the system peak load of 1.75 A and charge

load of 1.25 A, the smaller adaptor’s voltage drops until the output voltage reaches the DPPM regulation

voltage threshold. The charge current is reduced until there is no further drop on the output voltage. The

system gets its 1.75-A charge and the battery charge current is reduced from 1.25 A to 0.25 A. When the

peak system load drops to 0.5 A, the charge current returns to 1 A and the output voltage returns to its

normal value.

2. Using DPPM provides a power savings compared to configurations without DPPM. Without DPPM, if the

system current plus charge current exceed the supply’s current limit, then the output is pulled down to the

limit) and the voltage drop is large for maximum power dissipation. With DPPM, the voltage drop is less

same for both cases. The advantages include less power dissipation, lower system temperature, and better

overall efficiency.

3. The DPPM sustains the system voltage no matter what causes it to drop, if at all possible. It does this by

reducing the noncritical charging load while maintaining the maximum power output of the adaptor.

Note that the DPPM voltage, V

where

SF is the scale factor as specified in the specification table.

The safety timer is dynamically adjusted while in DPPM mode. The voltage on the ISET1 pin is directly

proportional to the programmed charging current. When the programmed charging current is reduced, due to

DPPM, the ISET1 and TMR voltages are reduced and the timer’s clock is proportionally slowed, extending the

safety time. In normal operation V(TMR) = 2.5 V; and, when the clock is slowed, V(TMR) is reduced. When

V(TMR) = 1.25 V, the safety timer has a value close to 2 times the normal operation timer value. See Figure 5

through Figure 6.

System Power

In this case, the system load is powered from a USB port through the internal switch Q1 (see Figure 4). Note

that in this case, Q1 regulates the total current to the 100-mA or 500-mA level, as selected on the ISET2 input.

load below the USB current level selected (if the battery is critically low or missing). Otherwise, the output drops

to the battey voltage; therefore, the system should have a low-power mode for USB power application. The

DPPM feature keeps the output from dropping below its programmed threshold, due to the battery charging

current, by reducing the charging current.

Charge Control

When in USB mode, Q1 regulates the input current to the value selected by the ISET2 pin (0.1/0.5 A). The

charge current to the battery is set by the ISET1 resistor (typically > 0.5 A). Because the charge current typically

is programmed for more current than the USB current limit allows, the output voltage drops to the battery voltage

or DPPM voltage, whichever is higher. If the DPPM threshold is reached first, the charge current is reduced until

the system.

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