Preliminary

Rev.01

13/16

EP1551

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During startup, P-channel switch transistor is always off, N-channel switch transistor is controlled by

startup oscillator’s output. N-channel switch transistor has fixed off time, its on time is determined by

startup current limit, inductor value, and input voltage.

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When OUTSU reaches 2.5V, startup is complete, and the main circuits begin to work.

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The oscillator’s frequency is determined by the RC network at OSC pin and the output voltage of Step-up

DCDC converter, so the frequency changes as OUTSU ramps upward following startup.

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Current sense amplifier (Isense AMP) senses inductor current when N-channel switch transistor is on.

When inductor’s peak current reaches 2A, N-channel switch transistor will be turned off.

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Reverse current sense amplifier (Izero AMP) senses inductor current when P-channel switch transistor is

on. When inductor current falls to zero, P-channel switch transistor will be turned off.

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If Step-up DC-DC converter has moderate or heavy load, inductor current is controlled by the output of

error amplifier (ERR AMP). With light load, Step-up DC-DC converter operates in idle-mode to improve

the efficiency. In idle-mode, peak current of every pulse is fixed at 200mA, and some pulses are skipped.

Step-down DC-DC Converter

Figure 9. Block Diagram for Current-Mode Step-down DC-DC Converter

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The structure of Step-down DC-DC converter is similar to that of Step-up DC-DC converter. Soft-start circuit is

used to ramp the internal reference up from 0V to 1.25V.