33730 datasheet(17/27 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33730

FUNCTIONAL DEVICE OPERATION

OPERATION DESCRIPTION

The direct voltage conversion to VDDH = 5.0V together with

the Protection FET Driver circuit allows operation of the IC at

very low battery voltages, which would otherwise require to

use a boost regulator (with an additional system cost) or a

different and more expensive switching converter topology

(e.g. flyback).

Short Circuit Protection

The switching regulator is protected against the over-

current and short-circuit conditions. It integrates a current

limit circuit, which has two threshold levels - the pulse by

pulse, and the extreme.

Pulse by Pulse Current Limit

Pulse-by-Pulse Current Limit threshold has a nominal

value set ILIM_SW. When the current flowing through switching

regulator power FET exceeds this value the power FET is

immediately turned off. During the next switching cycle the

power FET is turned on again until it is commanded off by its

natural duty cycle or until the current reaches the threshold

level again. It should be noted that the current limit is blanked

for several tens of nanoseconds during the turn-on and turn-

off transition times in order to prevent erroneous turn off due

to the current spikes caused by switcher parasitic

components.

Extreme Current Limit.

In some cases, during the over-current or short circuit

condition, the inductor current does not sufficiently decay

during the off time of the switching period. The current rise

during the current limit blanking time is higher than the decay

during the off time. In this case the current in the inductor

builds up every consecutive switching cycle. In order to

prevent the power FET failure during this condition an

extreme current limit has been implemented. When the

current flowing through the power FET reaches the

ILIM_SW_Ext threshold, the switching regulator will shut off for

500

μs, before the switching regulator is allowed to turn on

again (see Figure 5).

Figure 5. 33730 Current Limit

Soft Start

The switching regulator has an integrated soft-start

feature. During the soft-start sequence the duty cycle of the

internal power switch will be gradually increased from low

value to the regulation level. This technique prevents any

undesirable inrush current into the buck regulator output

capacitor.

LINEAR REGULATORS

The 33730 integrates two linear regulator control circuits

VDD3 (programmable), VDDL (programmable) both capable of

driving up to 15mA (min.) base current into the external pass

NPN transistors. The output voltage of both linear regulators

is monitored at their feedback pins (VDD3 and VDDL). If the

voltage at any of the VDD3, VDDL feedback pins fall below their

regulation level, the supervisory Reset control circuits will

assert the corresponding reset signal (RSTL, and/or RST3

lines will be pulled low). See Table 6 for the output voltage

selection details.

The linear regulators will stay in regulation down to 4.5V at

the KA_VBAT pin.

The 33730 linear regulators offer high flexibility and

variability of the module design in terms of selectable output

voltages as well as wide range of output current capability.

There several types of suitable external pass NPN transistors

which could be used. The choice of the particular type

depends mostly on the expected power dissipation of the

pass transistor. The following parts provide good solution and

have been bench tested with the 33730:

BCP68T1 (SOT-223)

NJD2873T4 (DPAK)

MJB44H11 (D2PAK)

Available from ON Semiconductor.

NOTE: The 33730 linear regulators have been designed to

use low ESR ceramic output capacitors - see Figure 8 and

Figure 9 for the recommended values.

STANDBY REGULATORS

The 33730 integrates two standby linear regulators, the

VKAM and the optional standby regulator VDD3 (see Figure 9)

for the optional standby circuit).The output voltage levels of

both standby linear regulators are programmable and

supervised by the Reset control circuits (RSTKAM, and/or

RST3). Both the VKAM and VDD3 outputs are capable of

delivering IVKAM_LIM and IVDD3_LIM of load current. See

Table 6 for the VKAM and VDD3 standby output voltage

selection details.

The VKAM standby regulator will keep functioning even

below VUVLO_f but the specified drop out voltage may not be

maintained.

NOTE: The 33730 standby regulators have been designed

to use low ESR ceramic output capacitors - see Figure 8 and

Figure 9 for recommended values.

PROGRAMMING LINEAR REGULATOR OUTPUT

VOLTAGE

The output voltage of the VDD3, VDDL and VKAM outputs

can be externally programmed by placing logic levels on the

programming pins P1, P2, and P3 (see Table 6). This

tBLANK

TSW

ILim

3.5A

4.5A

500us d elay

TSW

Inductor

Cu rrent

Switcher

FET Gate

Ext.

ILim

33730 Datasheet(PDF) 17 Page - Freescale Semiconductor, Inc

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